III.
5. Globular eyes. Cambrian, in the youngest zone, the Olenus schists. The oldest at present known oculate trilobite Eurycare is found in the second division of these schists.
In the lowest Lower Silurian division, the Ceratopyge limestone, Euloma and Ceratopyge occur as the last survivors of the blind, partially ridge bearing genera. The Trinucleidæ and Ampyx belong to another group of trilobites.
Even among other exclusively Lower Silurian genera, in which the plurality of the species is oculate, there are species entirely blind. So with Illænus, in which genus Dr Holm has not found any eye in Ill. Angelini, I. leptopleura and Ill. cæcus. The free cheek in these three species is much narrow, as the facial suture lies near the margin of the head.
The eyes of the Trilobites.
If, as is probable, to judge by the conformity of their cornea with that of recent crustacea, the trilobites like these were provided with crystalline cones beneath the corneal lenses or facets, only the latter have been preserved in a fossil state. Although the crystalline cones in consequence of their solid consistence might have been petrified as well as the cornea, they must, imbedded as these tiny cones lie, entirely wrapped up in delicate tissues, fall away and be lost, when the dissolution of the dead body had set in. Consequently the curious appendages on the inferior side of the lenses in Dalmanites vulgaris ([Pl. III f. 50]) or Phacops quadrilineata ([Pl. V fig. 38]) can noways be considered as belonging to the original structure of the eye, apart from their great dissimilarity with anything appertaining to the eyes of the Arthropoda. The cornea on the contrary cohered with the integument of the body, and it has been well preserved in a great number of trilobites.
I subdivide the trilobites in respect to the form of their eyes in the following manner:
I. Genera with compound eyes.
1. With prismatic plano-convex cornea facets.
Acidaspis, as a transitional form to the next group.
Asaphus.
Bumastus.
Cyphaspis.
Dysplanus.
Encrinurus.
Illænus.
Megalaspis.
Nileus.
Phillipsia.
Niobe.
Proetus.
Ptychopyge.
Symphysurus.
2. With round or biconvex transversally elongate lenses.
Acerocare.
Bronteus.
Chirurus.
Ctenopyge.
Cyrtometopus.
Eurycare.
Peltura.
Sphærophthalmus.
II. Genera with aggregate eyes of biconvex lenses.
Acaste.
Chasmops.
Dalmanites.
Phacops.
III. Genera with isolated eyes, one or several stemmata at
the extremity of a straight facial ridge.
Harpes.
Harpides.
(?) Trinucleus in the larval state.
Only a few authors have before now occupied themselves with the intimate structure of the trilobite eye. Packard gave in 1880, in the »American Naturalist» a note on the structure of the eye of trilobites (p. 503). There are some rough and inexact sketches of the eyes of Limulus and Asaphus, and although he seems to have known the beautiful researches of Grenacher he still »claims that the trilobite eye was organized on the same plan as Limulus». This statement is altogether wrong, and as I hope to show the trilobites have had eyes entirely different from that of Limulus and instead agreeing with those of the Isopoda and perhaps also with a few other Crustacea. In 1889 J. M. Clarke published an account[28] on the »Structure and development of the visual area in the trilobite Phacops rana Green». The aggregate eye described by him are of the type forming my third group. His holochroal division embraces my first and second groups and the schizochroal my third.
[28] Quart. Journ. of Morphology, vol. II p. 253.
The latest contribution to the knowledge of these eyes is found in Exner's »Physiologie der facettirten Augen von Krebsen and Insecten», 1891, where he gives good figures of the lenses of Phacops fecundus, pl. II figs 18, 19. He says that the palpable difference in the structure of these eyes and those of Limulus point to a change in the function of these eyes.
I. Compound eyes.
1. Eyes with prismatic, plano-convex lenses.
A pellucid, smooth and glossy integument, a direct continuation of the common test of the body covers the corneal lenses, quite as is the case in so many of the recent crustacea. In the plurality it is, however, difficult to discern the lenses from the outside.
The lenses, as seen in a vertical section of the eye of Asaphus expansus, ([pl. I fig. 12]), are columnar prisms, like the pillars of basalt, attaining a length of 0,2 mm and at the point where the eye joins the test of 0,3 mm. At their interior extremity they have a breadth of 0,066 mm. On that point the surface is convex and at the exterior surface plane. They are closely packed and in a transverse section resemble a pavement of regular hexaeders. But they also assume other shapes and become rhombs or even quadrates, as seen in a specimen of Asaphus fallax ([pl. I fig. 18]), where the hexaeders and quadrates lie side by side without transitional forms. As a rule the lenses become more and more irregular in the vicinity of the surrounding frame or near the suture, nearly blotted out, as it were, and without any definite border line mingled with the confused, spongy mass that like a belt or a frame surrounds they eye in Asaphus and is sharply limited from the other part of the free cheek. This remarkable zone which is almost only present amongst the Asaphidæ (Asaphus, Megalaspis, Ptychopyge, Isotelus) retains in a confused manner somewhat of the prismatic structure of the eye as shown in the section ([Pl. I, fig. 11, b]). The eye of Bumastus also is environed by a similar zone, with a structure like that of the eye ([Pl. II fig 35, 41]).
In an undetermined species of Asaphus the lenses, although somewhat apart, are of an elongated hexaedric outline, which passes into a regular circular one farther away and on the surface of the eye they are slightly convex ([Pl. I figs 27-29]). In other genera belonging to this group the shape of the lenses are like those of Asaphus, so for instance in Illænus (I. chiron and I. Esmarki) and in Niobe. In Dysplanus centrotus they are shorter and broad, and their interior or lower surface strongly convex. It is likewise so in Nileus, where Nileus armadillo has an exceedingly thick exterior integument above the lenses. Such an integument has in a still higher degree increased in Bumastus sulcatus so as to exceed in thickness the stratum of the corneal prisms and it may in fact be doubted if the eyes of this species ever were able to function as visual organs. Proetus nearly resembles Bumastus in the thickness of the integument covering the prismatic lenses, which are interiorly convex, with a diameter of 0,03 mm.
In all genera belonging to this group a horizontal section gives the image of the hexaeders as in Asaphus with some change to squares or rhombs.
In scrutinizing a horizontal, somewhat extensive section of an eye in this group of trilobites, it will be perceived, as for instance in the figures ([Pl. V fig. 16, 22]) that the regular and evidently homogenous and intact prismatic lenses by and by have been altered and in a part of the section, a little distant from the intact ones disintegrated in their interior, showing various aspects of alteration. I cannot but think that this is a destruction which has set in long after the fossilization. It has revealed certain states of the intimate structure, certain delicate details, that now with an astonishing regularity come in sight and probably also lie hidden in the intact prisms. In the specimens of Asaphus, which we have studied, the alteration has taken the shape of a concentric stratification forming the body of the prisms, which is well discernible in a horizontal section, but not easy to detect in the longitudinal one ([Pl. I figs 9-10, 11]). It is likewise so in Niobe. In the other genera again the decomposition makes the prisms look like empty tubes in which a few irregular traverses and trabecular remains of their solid mass radiate towards the interior. They thus assume the aspect of a composite coral with its septa in the calicles ([Pl. VI fig. 31]). This is also evident in Nileus palpebrosus and Dysplanus.
2. Eyes with biconvex lenses.
The surface of the eye is, as in Chirurus glaber Ang., a mass of contiguous hemispherical lenses, probably once covered with a membrane, as is still to be seen in well preserved specimens of Bronteus laticauda. Both in Chirurus and Bronteus the lenses seen vertically are globular and ordinated beside each other either continuous or separated only through a faint dividing line. In a horizontal section passing right through the point of contact they show the common hexaedral shape and when somewhat corroded the interior radiate structure also comes forth, the radii directed towards a little black point in the centre. The lenses of the Brontei have the same stellate structure as in Bumastus ([Pl. II fig. 7]). In Cyrtometopus the lenses are in size the fourth of those in Chirurus and they form an extremely thin stratum in strongest contrast with the adjoining cheek, which surpasses them more than six times in thickness ([Pl. III fig. 19]). The lenses of Cyrtometopus are more flattened and irregular than in the former genera. The free cheek around the eyes does not form a border zone, somewhat imitating the eye structure as in Asaphus, but is more compact, composed of vertical elements which give to the test of the trilobites in general a tendency to split up in vertical prisms.
Of a peculiar interest are the eyes in the oldest of all oculate trilobites, at present with certainty known, Eurycare, Peltura, Sphærophthalmus and Ctenopyge.
Of these genera Eurycare is the oldest (see table [p. 22]). Amongst the many free and detached cheeks only a single, very little one has been found with the eye ball fixed. It seems to be of the same structure as in Sphærophthalmus. In Sphærophthalmus and Ctenopyge the eye globes are enormous, considering the size of the cheek in which they are set and occupy more than a third of the length of the free cheek ([Pl. III f. 26, 31]). They are hemispheric, blackish and glossy, more so in the former genus. The spheroidal lenses, projecting on the surface, are in Ctenopyge larger near the facial suture and small at the opposite side where the eye is fixed in the free cheek. For the rest, in both genera ([Pl. III fig. 34]) the lenses form a thin stratum, where they in a vertical section lie elongated, flattened and biconvex, slightly joined with each other at the point of contact. The fine form which they exhibit reminds of the lenses of Sphæroma.[29] They are in diameter thrice as long as they are high. Seen in a horizontal section passing through the point of contact they show hexaeders with a curiously jagged outline ([Pl. III fig. 33]).
[29] Bellonci Atti dei Lincei. Memorie, vol. X, 1881, Sphæroma, pl. II fig. 11.
Peltura which is coeval with these, has a narrow semiglobose visual field ([Pl. III figs 35-41]), the superior surface of which is quite smooth and evenly rounded. On its interior side there stand out, somewhat distantiated, in a low relief semiglobular facets, quite as regular incrassations of the cornea, thus not forming free lenses, but rather reminding of the for the rest differently formed quasi-lenses of Limulus. In a vertical section they appear as the inferior moiety of real ovate lenses ([Pl. III f. 40-41]).
The much younger Acerocare has a similar cornea. A very little specimen, the head of scarcely more than one millim. in length, retains both eyes, of which one shows the slightly convex lenses and the other a cast of the interior side as in Peltura. These both genera should in consequence of their peculiar limuloid cornea be ranged for themselves apart from the real lenticulate genera, but any material sufficient for doing this properly, is at present not at hand.
II. Aggregate eyes.
These are found solely in the family of the Phacopidæ, unless the Lichadidæ were also provided with this sort of eyes, but we have had no opportunity to study them. It seems, however, not likely that they had aggregate eyes. Barrande has represented them quite as finely reticulate as the eves of any Asaphid. We have sectioned and figured the eyes of Dalmanites vulgaris and D. obtusus from the Silurian of Gotland and found that these have truly aggregate eyes, each consisting of a regular biconvex lens, lying enclosed in a socket of its own and covered by a cornea of its own. The distance between the eyes is much variable and in a few instances they are nearly contiguous. Extremes are seen on [pl. III figures 43, 47]. The lenses are comparatively large, and have always had a covering membrane, though this in many instances has been lost. This membrane which is an immediate continuation of the general integument of the body covers the lenses all round their superior moiety. In its prolongation downwards between the lenses ([Pl. VI fig. 3, 4]) it is free from the contact with them and hangs alongside and around much incrassated, so as to take in a section a lengthened lancet like shape. It lies thus alongside the other interstitial test, and is like this perforated by longitudinal canals. In a horizontal section taken a little below the surface it encircles the lens as a wall like ring ([Pl. VI fig. 1, 2]). In a vertical section the lenses lie in direct contact with the cheek without any intervening zone and the cheek has the structure so common amongst the trilobites, being perforated by vertical tubes going straight down from the surface ([Pl. III fig. 44], [Pl. VI f. 5]).
In Dalm. vulgaris and also in Phacops quadrilineata there is as already before mentioned a peculiar structure beneath the lenses, consisting of narrow, threadlike, straight lines, twice as long as the lenses ([Pl. III 49-50], [pl. V fig. 38]). In a horizontal section they are found to be irregular prisms closely packed. It can not be any structure peculiar to the eyes or the lenses, rather some parasitic growth added since the death of the animal. The lenses are in several specimens composed of clear calcareous spar. In others again they have been filled with a dark muddy calcareous rock excepting in the lower moiety where there is left a residue of the white spar, having in all lenses assumed a regular shape which I consider as organic ([Pl. VI fig. 5]). This spar covers the whole bottom and its upper rim is incrassated and bent inwards. In horizontal sections this residue is a whitish ring close inside the interior ring wall ([Pl. VI f. 2]). I would suggest that this curious conformation is due to the original structure of the lens, supposing that it in these crustaceans has been built upon the same plan as in several other Arthropoda. In Cymothoa[30] and in Sphæroma[31] for instance the lenses are built up of thin strata, which are parallel with the convex outside, so that on the inferior surface of the lens they are arched downwards and on the superior side upwards, being not strictly concentric. In the spiders they are constructed upon this same plan[32] perhaps more evidently. If now in Phacops the lens consisted of such semiconcentric strata and the upper moiety has been destroyed, the rest must have taken the shape as we find it. It is moreover peculiar that the destruction has been exactly similar in all lenses of that specimen. Can it be due to the circumstance that the power of resistance in the inferior strata has been greater?
[30] Bullar Philos. Transact. 1878, pt. II, pl. 46, fig. 12.
[31] Bellonci Atti dei Lincei, Memorie X 1881, pl. II, fig. 11.
[32] See Grenacher pl. II f. 18 Epeira.
In Phacops quadrilineata the lenses are more elliptic than in the former. On their interior surface beneath the spiny tufts mentioned large hexaedral prisms of clear calcareous spar issue, one prism for each lens ([Pl. V fig. 38]), having thus a very deceptive appearance, but no doubt of inorganic origin with the lenses as a basis for their crystallization, quite as in the Cystoids where the interior often is converted into a mass of crystalline prisms, issuing from the interior surface of the plates.
III. Genera with stemmata and ocelli.
In a little group that has retained larval or ancestral characters during a great part of the palæozoic period, the genus Harpes stands as a type. It has ranged from the oldest Lower Silurian, if we join the related Harpides, to the middle Devonian. From near the top of the glabella, though not so much forward as the facial ridge of the blind trilobites (Olenus, Liostracus etc.) a straight ridge of much varying length stands out on both sides and at its extremity two or three globular stemmata with glossy surface lie encased. Probably this ridge has the same origin as in the Olenidæ, the more so, as there are indications of an extensive circulatory system. On [plate IV fig. 18-19] the right hand ocelli of Harpes vittatus Barr. from Lochkow, Bohemia, are represented. They are two, lying isolated near each other, quite globular with circular outline, smooth and glossy as to exhibit a shining surface. Their size is 0,4 mm in diameter. Being cut vertically in the direction of the longitudinal axis of the head they resemble elongated hemispheres, convex on the exterior surface, slightly concave on the interior. The test of the head lies between them as a saddle and covers them only partially and on the outer sides they lie with their margins encased in the head shield. Seen in thin sections of the right lens and magnified the whitish mass is traversed by vertical, blacker streaks, standing somewhat radiating towards the sides and cancellate. The other lens has a horizontal row of black dots. All this is probably not of any structural value, only due to later changes.
The remarkable genus Harpides from the lowest Lower Silurian belongs also to this group and has beside the fixed peduncle a peculiar elongated ridge going from the eyes to the lateral margins of the head[33], a ridge which is also present in some of the true Harpes.
[33] Harpides breviceps cannot belong to this genus and is rather related to Erinnys Salter, as also Matthew holds it.
As Beecher has shown[34] the larva of Trinucleus possesses quite the same transverse ridge with intumescent eyelike extremities, and although the smallness of the specimens has not permitted to ascertain the presence of a true eye, it may be apposite to suppose it on the homology with the eyes of Harpes, a genus with which Trinucleus is related. But as well known, in the adult Trinuclei there is no trace of these ocular ridges nor of real eyes so that Beecher in his paper »Blind Trilobites»[35] numerates Trinucleus amongst these. Out of the nine Scandinavian species of Trinucleus no less than seven have a well marked little tubercle on each side of the glabella placed exactly on the same spot where the larval Trinuclei had their much larger eyes placed. In this case it may be allowed to suppose that the tubercles are the direct successors of the larval eye and that they are true ocelli. Reede seems to be willing to regard them as possessing a visual function.[36] Beecher[37] holds the eye nodules in the larva and the ocelli in the adult to be identical.
[34] Structure and appendages of Trinucleus.
[35] Geol. Magazine 1898. pp. 439, 493, 552.
[36] l. c. p. 447.
[37] l. c. p. 309.
But, as Barrande has shown,[38] there is a certain species of Trinucleus, the larva of which wants a facial ridge and eyes, as there also are several adult forms without ocelli. These have remained on a much ancestral stage, while the larva with eyes are more highly developed in such species, where the adult have been subject to a retrograde development. Ampyx and Dionide, though completely blind, evidently belong to this group, and once, as is to be hoped, larval forms may be discovered showing their development. In a certain way Arethusina shows characters proper for this group, in having the straight ocular ridge, quite as in Harpes, but eyes of the reticulate type and probably prismatic. It thus like Harpes conserved an ancestral characteristic long periods since it had disappeared in most of the other genera.
[38] Sys. Sil. de Bohéme I, pl. 30, figs 41-50.
Of the groups, in which Joh:s Müller[39] long ago classified the Crustacean eyes, his second »Hauptgruppe» (»Aggregate von einfachen linsenhaften Augen») and the fourth »zusammengesetzten Augen ... facettirte Hornhaut» the former corresponds with my third and the latter with my two first divisions. In so far as the cornea and its facets or lenses are to be regarded, there is the greatest analogy with the Isopoda. In vertical sections of Sphæroma we have the same sort of elongated, flattened biconvex lenses as in Sphærophthalmus and others. Since Grenacher and Exner and others have published their excellent works on the eyes of the Arthropoda, there can be no foundation for speaking of the resemblance of the trilobitic eye with that of Limulus, as this genus stands completely isolated amongst all Arthropoda in that respect. There is, as stated above, a certain resemblance between the cornea of Peltura and that of Limulus, but this is not yet ripe for a discussion. Nor is there any evidence for correlating the eyes of the trilobites with the eyes of the Phyllopoda. Bernard thinks that the so called eye of the Paradoxidæ has been formed upon the same plan as that of Apus. There is nothing to prove this hypothesis that the facial ridge or any part of it ever had been a visual organ, and the evidence at hand rather tends in a contrary direction.
[39] In Merkels Archiv 1829 p. 46 and in Treviranus Zeitschrift für Physiologie Bd IV p. 97.
There are signs of long physiological and anatomical efforts to prepare the development of the eyes on the free cheek, as revealed through the long series of blind trilobites. A system of radiating blood vessels, similar to those described above as covering the inside of the head in some older genera, all issuing from the scallop in the free cheek, where later the eye had to find its place, have left their stamp, their mark on the surface of the free cheek. They attest the great vital activity which was so intense at the point were the eye was to be formed. We give the figures of two such cheeks of different types. One from Parabolina spinulosa ([pl. V fig. 31]) is the more common, where six or more isolated trunks radiate from the semilunar ridge round the indenture and subdivide in branchlets which cease near the lateral margin of the cheek. It may be that it is an annular vessel near the indenture that feeds them all and that this probably is in connection with the great central circulatory system. In Olenus ([pl. V fig. 29]) the vessels are partly anastomosing and form a reticulate system and they are studded with minute wartlets. Another sign, which may be taken as a preparation, is the elevated rim around the scallop, which is so prominent in several of the Cambrian genera, but which does not embrace any facet bearing cornea..
From what has been stated above the following conclusions have been arrived at.
1. The plurality of the genera living during the Cambrian period were blind and it was first at the close of that period, in the Olenus schists, that genera with real visual organs appeared. There may have been oculate trilobites earlier, as Solenopleura, but we know nothing of their eyes.
2. The primordial glabellar pleuron which was metamorphosed into a facial ridge is no visual organ. It is in the Olenidæ nothing but the elevated line made in the test by the subjacent main trunk of the circulatory system. It swells out in a node, »palpebral lobe», but not before the facial suture has been formed. In the genera where there is no facial suture, there is no node. In the Paradoxidæ where the ridge is of a different origin, there is no node, though there is a suture.
The four types of eyes in the trilobites have probably succeeded one another in the following chronological order:
1) with stemmata or ocelli; 2) biconvex or lentiform; 3) prismatic; 4) aggregate. The oldest known representatives for each type are for 1) Harpides rugosus in the Ceratopyge limestone of the Lower Silur., for 2) Eurycare, in the Cambrian Olenid schists, division 2, for 3) Megalaspis, in the Ceratopyge limestone of the Lower Silurian, for 4) Phacops in the Lower gray Orthoceratite limestone. The eyes of the trilobites show the greatest conformity with those of the recent Isopoda.
The most perfect eyes amongst all the trilobitic eyes may be those of the Phacopidæ, which are also geologically the youngest, the least developed again those of the Proetidæ or rather of the Bumasti. The great thickness of the cornea in these must have weakened their power of vision and they had probably only a faint perception of light.
On the maculæ of the hypostoma.
We shall now turn our attention to the visual organs which Liljevall discovered on the hypostoma of Bronteus. In doing this, we may bear in mind, that the genera in which we really have found lenses on the maculæ are relatively few, but that we shall review the maculæ in all genera, which we have been able to examine, and try to show that even most of these may, although in an inferior degree, have acted as visual organs. At the same time some more details shall be given about the cephalic eyes for comparing them with the hypostomic ones.
Acidaspis Murch.
The hypostoma is of a peculiar type, deviating from that of the other genera, squarish or rectangularly transverse, entirely without terrace lines and no maculæ proper. Barrande has given no less than nine different samples of these hypostomas.
Acidaspis crenata Emmr.
There are certainly no maculæ of the same sort as in so many other trilobites and the interior side of the hypostoma does not bear the least traces of impressions which might be taken as the reverse of the maculæ and still more less as muscular impressions. In fact, the total absence of such in Acidaspis militates against the interpretation of the macula impressions as muscular scars. When the maculæ fail, also the so called muscular impressions fail. Moreover we are able to see still clearer in this matter through what we know about the structure and position of the extremely similar hypostoma of Apus, of which we have given a description in the end of this memoir. This skeletal part of Apus is without any connection with the surroundings, excepting at its anterior margin, and is consequently movable in a direction outwards and upwards. Along that anterior margin it is fixed to the outside of the ventricle through three pair of muscles, three muscles on each side of the margin. Zaddach De Apodis cancriformis anatome (pl. II fig. XIV p. 68).
For the rest the marks of the attachment of the muscles are as a rule in the Crustacea, at least those of the head, elevated small platforms, so in the Trilobites, of which I have excellent specimens in a Bumastus and others. On the inside of the head of Limulus they are faintly elevated patches. The small hollows on the inside of the hypostoma formed by the maculæ are indeed the sockets in which the soft parts of these more or less developed hypostomic eyes were sheltered. But there is still a feature in the hypostoma of Acidaspis which merits our attention and which perhaps may have a significance akin to that of the maculæ.
The hypostoma is square, with two short pointed wings, one on each side of the slightly bent anterior margin and likewise two smaller ones at the corners of the posterior margin. A groove follows on a short distance the lateral margins and the posterior margin and disappears a little below the anterior one. In the same direction, distally, though a little more inside and unconnected with them there are two small grooves, the bottom of which consists of a shell substance of different colour and structure than the other parts. Having been a little ground and seen in transmitted light it exhibits the shape of a club and a homogenous yellow spot, tapering posteriorly and swelling out distally ([Pl. I fig. 4]). It must be left an open question whether these maculæ share in the nature of visual organs as the quite different maculæ of the other trilobites, but it may be possible that it is so. It must, however, be remarked that there are two types of hypostoma in the genus Acidaspis as shown by the illustrations of Barrande. One has the small grooves, possibly all sheltering the claviform maculæ, disposed as in the now described A. crenata. This group embraces five species of the Bohemian Silurian formation. The other group of three species again has the hypostoma of the same quadratic or rectangular shape, but the two short grooves, which may be expected to contain the maculæ, are placed midways between the anterior and the posterior margins, nearly as the maculæ bearing grooves of other trilobites. We have however not had material for pursuing our researches in this genus, the other species of the Swedish Acidaspidæ being unknown as to their hypostoma.
It may here be added an observation concerning the ornamentation of the exterior surface of the hypostoma of Ac. crenata. It is covered by a great number of diminutive circular or oblong wartlets occupying the whole surface excepting the lower third of the central field just above the posterior groove which is smooth. These wartlets seen through transmitted light ([Pl. I fig. 5]) show in their interior something like a peculiar black spiculum rising from a bifid rootlet and confined within the wartlet and not extruding from it. In a longitudinal section ([Pl. I fig. 6]) the spicula perforate the wartlets reaching through their whole length. As in Calymmene these interior pseudo-spicula are tubes, filled with iron-pyrites. It is probable that these tubes were once bearing setæ and quite as in Apus formed a fur of bristles.
The structure of the cephalic eyes ([Pl. 1 f. 1-2]) is prismatic, but the separate prisms are rather short and broad. Their lower or interior end is convex. The separating lines between the single prisms are not always distinct.
Agnostus Dalm.
Agnostus glandiformis Ang. Although there is not the slightest evidence of eyes in this the largest of its genus, nor any free hypostoma hitherto has been found, we may here give a little account of our researches into this species. The scantiness of material forms a chief obstacle to our knowledge. Only three entire, rolled up specimens have been found, and it is by sectioning and preparing such that any hope can be entertained to gain reliable results.
A rolled up specimen from Andrarum Scania was sectioned lengthwise. The tail-piece closed tight against the head-shield, so there had been little chance for foreign matter to penetrate into the interior which, however, is filled. Close below the cephalic shield there is a remarkable structure, mostly resembling an elongated intestine with swellings joined by more narrow ducts and anteriorly the coherence is interrupted. As in crustaceans and Arthropods in general the stomach and the intestine are situated on the dorsal side of the body, there is nothing unlikely in assuming that this in reality may be the remnants of the intestine. This may also be compared with the observation made by Vollborth in his memoir »Ueber die mit glatten Rumpfgliedern versehenen Trilobiten», 1863, p. 46, tab. 1 fig. 12 where the heart-tube probably is delineated. Barrande has also given figures of what he considers as the intestine in Trinucleus.
Below this organ in Agnostus, there lies a section of a vaulted calcareous plate with its convexity turned against the dorsal side of the head shield, that is to say quite the reverse what might have been expected if it had been the hypostoma in its true position. This may, however, not be any objection against considering it as a sectioned hypostoma, loosened from its connection with the cephalic shield, disturbed in its original position and turned round, when the shell became filled with mud. At the distal end there is a much distinct duplicature.
Asaphus Brongn.
The wellknown hypostoma of this genus has the same characteristic shape, though more pronounced, as in Ptychopyge, with its posterior margin deeply indented, so as to form two large, pointed lobes. There is properly only one median field, surrounded by flat, lateral borders, continuing down into the posterior lobes, from which it is separated through a shallow groove. The macula are situated in this groove, on each side of the inferior border of the central field. They are more or less prominent, but whatever their form may be, their surface is always entirely smooth, lying well circumscribed amidst the surrounding terrace lines.
The following list enumerates all the species in this genus, of which previous authors have delineated the tubercles, though they in the descriptions only in very few instances have mentioned their presence.
A. acuminatus Nieczkowski, »Zusätze zur Monogr. der Trilobiten 1859», tab. I, f. 6. Maculæ most prominent, but no mention made of them in the description.
A. (Isotelus) canalis J. Hall, Pal. N. Y. I, pl. 4 bis, f. 18-19. Though this and a following species probably on account of the deviating conformation of their body, belong to a different genus or subgenus, I mention them along with the Asaphi, as there is the greatest similarity in their hypostoma. Fragment of an interior cast; still more strange is the fragment showing part of the interior surface and the duplicature.
A. expansus L. Br. I pl. VII f. 3, BR. II pl. I f. 2, 2 c. In the two first figures there are no tubercles nor maculæ marked, in the last figure there are distinct tubercles.
A. fallax A. pl. XXVIII f. 3, c. BR. II pl. I, f. 3, both nearly congruent.
A. (Isotelus) gigas J. Hall, Pal. N. Y. 1, pl. 60, f. 7 g, pl. 66 f. 5, the inside of an entire, uncommonly large hypostoma gives a general good view with the macula; which are large and evident. He mentions the maculæ as »two circular spots». »These probably indicate the points for the attachment of muscles and tendons upon the inside,» he adds.
A. ingens Barr. Novák II, pl. I, f. 7, a good figure showing two semiglobular maculæ. Barrande's figures pl. 33, f. 7, 8 are not distinct.
A. ludibundus Tqt. Br. II, pl. 1, f. 7. Two semilunar sulci on the cast of the interior surface, being only the posterior borders of the maculæ which are a little more raised and distinct than the rest.
A. nobilis Barr. pl. 32, f. 6. Indistinct traces of maculæ. In the figure 6, pl. 31 representing the hypostoma of a young specimen, there are no tubercles at all. If the figures were to be relied upon, it might be assumed, that the tubercles appear at a more mature age.
A. Powisii Salter, pl. 23, f. 6. Two semilunar, narrow tubercular maculæ, their interior apices converging towards the anterior border of the hypostoma.
A. raniceps A., pl. XXVIII, f. 2 c., BR. II, pl. 1, f. 4. A good figure.
A. raniceps var. maxima Br. II, pl. 1, f. 6. Semilunar impressions on a cast and thus far incomplete.
A. striatus Boeck. To this belongs probably A. expansus M. Sars »Ueber einige neue oder unvollständig bekannte Trilobiten» in Isis 1835 p. 333, and especially p. 340 and the following (Bemerkungen über die untere Seite von einigen Trilobiten). Sars there gives a very good description of the exterior side, where he also mentions the two maculæ, pl. IX, f. 9 a, b. (»2 kleine Knoten»), Br. I, pl. VIII, f. 4 a, and Br. II, pl. I, f. 9. These latter figures differ in so far, that the former has the maculæ excavated or rather tubular below, in the latter again they are regular.
A. trinucleorum Br. II, pl. I, f. 16 a large specimen with linear maculæ arranged rectangularly in respect to the longitudinal axis of the hypostoma and both on the same level.
A. tyrannus Murch. Salter in Mem. Geol. Survey, Brit. Foss. Dec. II, 1849 pl. V, f. 4, excellent figure. On p. 2 he says: »there is an oval circumscribed tubercle at the origin of each (fork) most distinct on the inner surface». This figure is again reproduced in the »Monograph», pl. 22, f. 6, where it is said »Two linear tubercles with their interior apices converging towards the posterior margin of the hypostoma».
Asaphus sp., probably A. raniceps, Pompeckj, Trilobitenfauna Ost- and West-Preussens, taf. VI, f. 7. p. 80 »an seinem Hinterrande liegen zwei kleine, flache Höckerchen, die als Reste des Hinterlappens aufzufassen sind»(!?).
We shall now describe the Asaphi which we have been able to examine more in detail.
Asaphus expansus L.
In the numerous specimens of which we have examined the hypostoma, there are always small, nearly circular maculæ which by their lighter colour are marked out from the surrounding smooth space of the inferior sinus of the lateral grooves where they are situated. They do not rise at all above the surrounding surface from which they are separated only by a fine, scarcely perceptible line. On the interior surface of the hypostoma they are better seen, and the enclosing line is deeper and more distinct. Their surface is there quite as smooth and even as on the exterior side of the hypostoma. They do not consequently in the least manner resemble muscular impressions as observable on the inside of the glabella of other trilobites. These are on the contrary elevated above the surface and finely striated in various ways.
The peculiar nature of these macula is revealed through the various sections we have made. In [pl. I fig. 15] a vertical section across the entire hypostoma is represented. The sectioned, dark maculæ (a) lie on both sides of the faintly curved central field, in the sinus of the lateral grooves, and the strongly developed terrace lines continue sidewards just a little on the interior side, where the duplicature turns round. As seen in a magnified vertical section ([fig. 16]) the maculæ consist of horizontal, whitish, straight lines, probably lines of successive growth, and these are crossed rectangularly by more irregular whitish lines separated from each other through dark spaces. The whole thus gains the aspect of a dark surface cancellated by white lines. This reticulated or spongious macula is enclosed as to its superior as well as to its inferior part in the compact and homogenous test of the hypostoma and occluded from the influence of the light. Its value as a visual organ consequently is insignificant. In a horizontal section ([fig. 17]) the structure is still more bewildering. There the whole macula is a confused, spongious white mass with dark spaces between the white meshes, and only at one side, the left one, some obscure indications as of polygones are visible. It would indeed have been impossible to interprete what this means, had not the study of the cephalic eye given a clue thereof.
The eye of this species is covered with a delicate perfectly smooth and glossy integument ([f. 12]) which is reposing immediately on the prismatic lenses of the cornea. In a few instances, depending on the colour or the state of preservation of that integument the lenses are indistinctly translucent. Owing to their state of preservation their aspect is greatly variable. They are in many instances, as seen in fig. 9, six-sided or some rhombic, foursided or even quadratic ([f. 8, 10]). They are all of the same length, 0,2 mm. along the surface proper of the eye, but are lengthened to 0,3 mm. towards the border of the eyes, [f. 12], which will be described further on; they are of equal breadth, amounting to 0,066 mm. Their inferior ends are slightly convex or nearly plane when well preserved, else, when as often is the case, disintegrated as to be scooped out and vaulted. Their interior structure, as revealed by sections, shows a cylindrical core, [f. 8-10], composed of concentric strata. This cylinder fills nearly the whole interior space of the prisms, there being, however, in many instances a compact dark mass between the cylinder and the walls of the prisms. There are also sections in which the prisms are filled with a uniformly black mass without any concentric structure. In longitudinal sections the outlines of the individual prisms are not clearly discernible ([f. 11]). There are longitudinal, white lines of varying thickness with lateral irregular offshoots, which may join with those from opposite walls, and give the interior a sort of spongious or cancellate appearance.
Towards both sides of the ocular surface, towards the superior and inferior side, a change sets in as to the shape of the lenses, as best seen in horizontal sections ([figs. 8, 10]) they are lengthened and become more and more indistinct, and at last in the upper and lower marginal zones pass over into a reticulate, spongious mass, which seen in a longitudinal section presents almost the same aspect as in the regular prismatic surface of the eye. [Fig. 11 b], [a] being the visual field. It is, however, more densely reticulate, but a prismatic arrangement is quite as much evident as in the ocular surface proper. In the rule the passage from the prismatic surface to the reticulate is gradual, the prisms becoming by and by irregular in their outline and diminishing in size ([fig. 12, a] the eye proper, [b] the border zone), but there are also instances where the distinction between these two fields is sharp and without any gradual transition. In the inferior reticulate zone there are generally some oblongue, funnelshaped pits. I am uncertain whether they are to be regarded as regular parts of the eye structure or rather as burrows of some parasite. They do not continue deep down.
If we now compare the reticulate zone of the eye with the maculæ of the hypostoma, for instance the vertical section [fig. 16] with [fig. 11 b], we find the most complete identity in structure. In the same manner the horizontal section of the border zone of the eye, [fig. 8 a], [fig. 10 b], and of the macula, [fig. 17] are similar. The same chaotic, spongious mass in both, with some tendency to form prisms more evident in the longitudinal sections, where the same reticulate structure with predominant white, longitudinal streaks is so palpable. That there is a complete identity in structure between the two, the macula of the hypostoma and the border zones of the eye, is as evident as anything can be, but as to the functional identity or what this function may have been it is difficult to decide anything with certainty, at least it seems to me that the capacity of vision must in both have been far more restricted than in the eye proper. They rather give the impression as of rudimentary visual organs.
Asaphus raniceps Dalm.
The maculæ of the hypostoma are placed obliquely in the saline manner as in the preceding species. They are more prominent and the oblong macula, with the longest diameter of 1,2 mm. is on the top of a little mound and surrounded by a fine, elevated marginal line ([fig. 23]). Its somewhat convex surface is entirely smooth and if sectioned horizontally exhibits the same sort of spongy texture as A. expansus ([pl. I f. 24]). In a vertical section ([fig. 25]) the macula does not occupy so large a space as in As. expansus, but rather lies as a lenticular disk in the hypostomic test closer to the superior surface than the inferior. With sufficiently high power the same sort of pillars, divided by horizontal strata is seen. Upon the whole the vertical section is not so clearly developed. The reticulate zone of the eye is more definitely separated front the prismatic zone than in A. expansus. There can be no doubt that there is a correspondence in this species between the structure of the macula! and the spongious zone of the eyes.
Asaphus cornigerus Schloth. (A. Kowalewskyi Lawrow).
Of this strange species with its enormous eyestalks, more than 2 centimeters in length I have through the kindness of Akademiker Friedr. Schmidt in St. Petersburg had occasion to study some specimens.
The rather large maculæ, obovate-circular, are like those of several other Esthonian species oriented inwards and upwards instead of inwards and downwards as in the Swedish species described. As the specimens have suffered through corrosion of the surface there are only faint traces of a marginal line. Their microscopic structure is badly preserved, and they look pale and transparent with only few indications of the spongious texture. The lenses of the cephalic eye are nearly square prisms sometimes with a slight approach to hexaedral pillars. There is no clear transition into a spongious or reticulate border zone. But this may depend upon the bad preservation.
Asaphus fallax Dalm.?
It is very difficult to distinguish this species or rather variety from A. expansus, but if we have found genuine specimens, there are some points in the shape of eyes and maculæ which make it different. The hypostoma ([f. 21]) is rather more broad and the duplicature is large with an upturned margin. The little macula is placed on the top of a smooth rounded elevation and enclosed by an elevated rim.
The cephalic eves are rather short, regular hexaedral prisms ([f. 18-20]) and as shown in [fig. 18] change into irregular squares near the border of the eve.
Asaphus sp.
from Brunsby kanal, Segerstad parish, isle of Öland. The maculæ are large pale whitish and of an uncommonly fine-meshed reticulation, well limited from the surrounding hypostoma.
Asaphus sp.
from the islet of Sandö, north of Gotland. Although we cannot give any account of its hypostoma, not having had sufficient material, the structure of the eyes is so peculiar that it seems worthy of being recorded. The integument is extremely thin and transparent and the subjacent lenses are clearly seen, and through their impact on the thin integument they make this to stand out in a very low relief above them ([fig. 27]). These lenses are uncommonly short, forming at the surface rather oblong, slightly hexaedral prisms with a narrow interspace between them. In a section lower down they have the shape of hollow, white rings filled with black mud and in a longitudinal section the white walls of the lenses look like short pointed spikes and interiorly they are completely empty. We here find also the same gradual change from regular cones to the spongious border zone as in the above mentioned species [f. 29]. The border zone is finely reticulated.
Asaphus (Isotelus) gigas J. Hall.
The enormous hypostoma of this giant resembles in a high degree that of the Asaphi, but is at the same time the most evident verification of the experience that the hypostoma per se cannot be regarded as the sole criterion for determinating the generic affinities of different species. The whole structure of this trilobite in other respects gives it a quite independent position, distinct, from Asaphus.
We have had at our disposal several specimens of the large hypostoma, the dimensions of the largest, fragmentary in its anterior margin, being as follows: breadth 48 mm. length 41 mm., probably 47 mm. when entire, breadth of each of the posterior lobes at their bases 19 mm., length of the same 23 mm. The maculæ which are placed on the flat surface of the hypostoma without being at all elevated, are prominent through their great size and their pale, whitish colour. They are somewhat oblongue having their longest axis directed inwards and downwards. They attain a diameter of four millimeters. Horizontally sectioned, [f. 2], they show the spongious texture and vertically the quasi prismatic reticulate structure richly developed, [f. 3]. The pillars are very distinct and continue without interruption all through the macula and in the interstices there are traverses joining between two pillars or partially filling the darkish interspaces, thus giving the whole the aspect of some »tabulate» coral sectioned. By the inspection of the horizontal section alone, presenting the irregular spongy texture it would have been impossible to imagine the ordinated arrangement which the vertical section reveals to us. The eyes consist of regular hexaedral prisms, and there is a very sharp boundary line between them and the spongious zone, which is very narrow and without distinct separating lines joins with the cephalic test. For the rest there are in all probability at least two different species sent from America under the name of Isotelus gigas, of which only the largest, almost like a Homalonotus, and the hypostoma of which has been partly delineated in this memoir, seems to be the real one. They differ both in the shape of the hypostoma as well as in other respects.
Barrandia Mac Coy.
According to Salter Monogr. pl. 19 fig. 9 his B. Portlocki, of which there is a fragmentary and broken hypostoma, shows feeble traces of oblique macula, nearly in the fashion of Asaphus. In the description at page 139 is told about »the usual pair of tubercles».
Bronteus Goldfuss.
Barrande delineates the maculæ on the hypostoma of
B. palifer, Tab. 45 fig. 17,
B. planus, Tab. 48 fig. 7, and
in the supplementar volume of
B. rhinoceros, Tab. 9 f. 16,
B. furcifer, Tab. 11 fig. 16, but there is not the slightest indication of granulation on any of them, nor is there in the descriptions, generic or specific, the least mention made of the tubercular maculæ. It is also remarkable that in the works of Angelin and Novák where several hypostomas belonging to species of this genus are delineated, not a single one shows these tubercles. We shall now continue the descriptions of the Swedish Brontei, already begun with Br. polyactin in the introductory part of this memoir.
Bronteus irradians Lindstr.
has a hypostoma that much resembles that of Br. polyactin. In its general shape it is similar and the two concentric grooves with the two maculæ placed in the same way, just below the superior groove. These maculæ are much larger than in Br. polyactin, nearly thrice their size. They are also more ovate or rather like a bean, the smooth surface is larger and the granulated spot restricted to a more narrow space forming an oblique patch. The granules or lenses are also individually larger than in the allied species, double their size or 0,06 millimeters. We have not succeeded in making sections of the cephalic eye nor of the maculæ.
Br. platyactin Angelin.
The hypostoma has a transversally triangular form, and is divided only in two fields through a shallow semicircular groove near the posterior margin. The two maculæ are situated above the groove near its superior sinuses. They are elongated, [fig. 17], elliptic with the narrow pointed end directed outwards and the broad rounded end inwards. The chief surface is scooped out as a shallow depression. The granulated spot is situated on the broader end and covering it completely. The relatively large lenses are arranged in five regular rows, the uppermost one being the longest. On the interior surface of the hypostoma there are the corresponding sockets of both maculæ with smooth surface. The horizontal sections of the granules [figs. 18, 19] present the image of white rings in close contact, without, however, to occasion a prismatic structure, a dark interspace lying between each ring. These lenses are filled with a dark mass, and in some the same sort of radiated structure is perceptible as in the lenses of the cephalic eye. In horizontal sections of the cephalic eye the lenses approach the polyedral shape. In another section near to the surface of another specimen the lenses are decidedly hexaedral. The vertical sections, [fig. 14], reveal their real nature as lenses where they lie as a string of beads with a dark nucleus enclosed within a thin whitish shell. They are covered by a thin membranous lining. When seen in transmitted light the lenses proper are dark, and the shell white and in reflected light the lenses are lighter than the rest.
Br. laticauda Angel.
Lower Silurian from Dalecarlia. The hypostoma, [f. 10], is of a broad clypeate shape, the anterior margin rounded without any large projecting wings. On the exterior surface there are two grooves parallel with the rounded inferior margin. As in Br. polyactin the two elliptic tubercles are situated on the inferior edge or slope of the upper groove and they are deepened by a shallow depression as in Br. platyactin. The granulated spot situated along the posterior margin of the macula deviates much in shape from that of the other species. It consists of a long and narrow stripe ending in a fine point outwards and widening inwards, where it is rounded, forming thus a claviform, curved elevation. Around the granulated area the maculæ are quite smooth, and show in a horizontal section an irregular structure of tiny black dots nearly resembling the structure of the hypostomic shell. We have not been able to obtain any good vertical section, but by casts of the interior side of the tubercles it is found that the granules form polyedric facets like those of the eyes though perhaps not so regular. There is a specimen with the granules intact, and the polyedric shape is then not so distinct.
Sections of the eyes elucidate the structure, which partially is obscure in other species of Bronteus. There is a thin membranous coating covering the subjacent well formed lenses ([fig. 8]). When this membrane in some instances has been peeled off the lenses lie bare ([fig. 9]). These have a dark nucleus and in some instances it seems as if there were two. In a vertical section they look spheroidal, in a horizontal section again they are polyedric, especially when taken somewhat below the surface or near the middle line ([fig. 6]). In [fig. 7], some lenses are delineated in a horizontal section, highly magnified and the corallian appearance is evident. The lenses are well separated by distinct lines, and from their inner tubes whitish reticulations radiate towards the centre.
If we now compare these sections, where the lenses are so distinctly seen, with the more obscure sections of other Brontei, it is evident that the dark points in them are nothing but transformed and deteriorated centres of the lenses. In Br. platyactin they are a little more distinct than in the other species. And it is with these that the granulated spots on the maculæ of the hypostoma are most concordant.
Bronteus sp. indet.
This species, of which only the hypostoma is known, found at Lansa, Gotland, is likely, to judge by that, to be nearly related to Br. polyactin. The shape of this hypostoma is exactly the same and the maculæ are placed in the same position, but they are larger, a little sunk on the blind surface and the group of the lenses is different. Its superior margin forms an ingoing arch and the lenses themselves are not convex, but rounded or slightly polyedric, and separated through thick interspaces.
Bumastus Murchison.
The only recorded hypostoma with maculæ belongs to B. insignis Hall as described by Salter Monogr. p. 208 pl. 27 fig. 7. He says: »A pair of compressed tubercles occurs at the lower third: they are transverse-ovate, and more than their own diameter apart.» This is nearly in concordance with what is seen on the species to be described below.
Although it can in reality be said that the species of Bumastus are the most common of all trilobites in the Silurium of Gotland, in so far that their head pieces and pygidia are met with everywhere in the limestone rocks of the island, the find of entire specimens or of detached hypostomas is amongst the rarest events when collecting there and amongst thousands of fragments, in several places entirely filling large portions of the limestone rock, there have in the Swedish State museum been acquired only three or four hypostomas, which we are now going to describe.
At first we must point out a certain resemblance which prevails between the hypostomas and maculæ in Bumastus and Bronteus. Compare for instance Bum. sulcatus with Br. platyactin and Br. laticauda. The general form is nearly the same in both and the shape of the maculæ almost identical but for the complete want of granulations on those of Bumastus. To this resemblance must be added that there are, as known, forms of trilobites which by some authors have been regarded as Brontei and by others, again, ranked amongst the Illænidæ.
Three species of Bumastus have been recorded as found in the Gotland strata, but of one of these no hypostoma is known. Besides, to judge by the head shields and hypostoma, there are a few new species. To begin with the most common
B. sulcatus Ldm.
The hypostoma is broadly shield-formed and the anterior margin is most characteristic being in its central part elongated in a short evenly rounded projection. The side wings are triangular acuminate, bent a little backwards and towards the interior surface. There is only a single rounded field passing into an evenly bent border, in front of which the projecting, elliptic maculæ lie. A second pair of wings stretches backwards near the posterior margin. We have not obtained any sections of the maculæ.
The cephalic eyes [figs. 36-38] are covered with an uncommonly thick stratum of homogenous shell, one and a half times as thick as the subjacent lenses. These are comparatively broad, above passing indistinctly in the covering stratum, below, at their basis, convex. In a horizontal section [figs. 33, 34] they exhibit the common hexaedral or pentaedric aspect, their exterior sides being white and confluent without any separating lines. Their interior is dark and from the polygonal sides white lines go in giving the whole the appearance as of a starry, composite coral with its septa. The correspondence in structure with Bronteus laticauda [pl. II fig. 7] is striking. A narrow space separates the surface of the eye from a nearly similar patch below the eve on the cheek [35 b, 38 b]. But there the position of the composing strata is singularly reversed and the spongious or pseudo-prismatic stratum lies close to the outside and the homogenous stratum on the inside. The white longitudinal lines are well marked out and regular in the vertical section, [fig. 38 b], indicating the prismatic structure most clearly.
B. barriensis Murch.
The hypostoma [fig. 31] is of so transverse a shape that the breadth much surpasses the height, the former being 6 millim., while the latter attains only 3,5 millim. The anterior margin is so much arcuated and sloping backwards that it forms an obtuse angle. The side wings are triangular and acuminated and from them a thick pad runs backwards and forms the posterior margin. Below the largest field of the exterior surface the maculæ rest on a ridge having a much elongated, vermiform shape and sunk in the surface ([fig. 32]) a little more than three times longer than broad. In its shape it approaches much to the hypostoma of Bum. insignis Hall as delineated by Salter pl. 27 fig. 7.
In another, undetermined Bumastus ([pl. II figs. 47, 48])[40] the hypostoma (from Klints, Othem, isle of Gotland) is of a more elongate obovate form, the anterior margin elevated, rounded, the terrace lines faint, the tubercles are larger than in the former Bumasti, oblong, curved, beanshaped, evenly rounded and smooth.
[40] Probably the same as Bum. sulcatus pl. XII fig. 12 in my paper on the Trilobites of Gotland 1885.
Another of the new species is one that Liljevall has found at Korpklint near Wisby ([Pl. II figs. 41-46]). The hypostoma is more elongate than transverse with the anterior margin regularly arcuated, by far not so prominent as in B. sulcatus and the former. By a shallow groove the exterior surface is divided in two fields, the anterior one the largest and covered with a few, distantiated, concentric terrace lines. The posterior field is smooth, nearly even and the maculæ lie just in the groove, with their axis oblique to the longitudinal axis of the hypostoma, beanshaped and smooth. The cephalic eyes show in a horizontal section hexaedral white walls enclosing a dark space in which some indistinct radii emanate, [fig. 41 a]. In the vertical sections the prisms are narrow and elongate and the integument by far not so thick as in Bumastus sulcatus, [fig. 42, 43]. The border zone of the eye in this ([fig. 41]) as well as in B. sulcatus, as seen in horizontal sections, agrees with that of the Asaphidæ.
Bumastus sp. inlet.
In a detached piece of limestone, found in Gotland, at Norderstrand, near Wisby, two hypostomas and a free cheek lay embedded along with a coral of the genus Acantholithus and a Bronteus probably belonging to a new species. As the coral has not been found above the uppermost beds of the Lower Silurian it is probable that the trilobites also are derived from the same horizon.
The hypostoma, figured [pl. III fig. 1], is broadly tongueshaped, with a short, blunt, triangular wing on each side of the anterior margin. The exterior side is nearly completely occupied by a single large field, which is convex and decorated by sparse terrace lines. Below this a smooth, nearly even plane, reaching to the inferior, elevated border of the hypostoma. The two tubercular macula' are situated exactly on the boundary line between the convex and the plane field. They are elongately ovate, pointed outwards, rounded inwards, with smooth and glossy surface. That portion, on which in the species of Bronteus the granular spot rests, is transversally wrinkled by some faintly elevated, sigmoid lines ([fig. 2]). It would have been of great interest to investigate the interior structure of these curious maculæ, but the scarcity of the material has prevented our doing so.
The other hypostoma, [pl. III fig. 3-5], is larger and has the terrace lines more distantiated and forming more open curves, but for the rest it is of the same shape as the smaller, so that we may not ascribe these small deviations to a specific difference. The maculæ are more elongated, narrow, curved tubercles, thus differing from the next preceding. A section running through its left tubercle does not show any structure, but the shell of the maculæ is extremely thin, thus contrasting strongly with the surrounding thicker shell, [fig. 4 a]. The cephalic eye, again, remaining on the free cheek found in the same piece of limestone and probably belonging to the same specimen, has its surface well preserved and exhibits semiglobular lenses in low relief, [fig. 5].
Calymmene Brongn.
In the following species of this genus hypostomic tubercles have formerly been recorded.
Cal. Blumenbachi Salter Monogr. pl. 8, fig. 9. The identity of the species is dubious.
Cal. duplicata Salter Mon. pl. 9, fig. 22. The tubercles rather more linear and approximated than in the subsequent. Salter calls them »two small transverse lobes ... forming a nearly continuous ridge».
Cal. tuberculata Angel. Tab. XIX fig. 5 c from the inside.
It is remarkable that the eyes or that part of them where the cornea and the lenses should be found, has been destroyed and entirely lost in all specimens of the Calymmene known to us, many hundreds having been searched and every one with a lacuna on that spot. It must have been of an excessive thinness. We know only of one instance, the Bohemian Calym. Arago, in which Barrande has seen the eye intact and provided with a small number of about eleven sphærical lenses.
Calymmene intermedia Ldm.
The clypeiform hypostoma is divided in two fields, one anterior larger, in the centre elevated in a short, blunt knob, and a posterior, bearing the two elevated rounded maculæ, just below the semicircular groove which separates it from the anterior field. These fields are surrounded by a large border which is posteriorly emarginated through a short rounded sinus. The surface is entirely covered by small semiglobular wartlets perforated by a straight pore or duct, which being filled with a black mineral, probably iron pyrites, gives a peculiar and characteristic aspect to the sections. In the same manner the whole mass of the hypostoma is pierced by straight, black lines, also filled up pores, starting from the outside and ending before attaining the inner surface.
The maculæ are prominent, separated from the hypostoma through narrow, distinct grooves, ovate, with the pointed end directed obliquely outwards, against the margins of the hypostoma. They are covered with wartlets and leaving only a little oblong spot free, quite smooth, this being consequently the macula proper. But in a longitudinal section there is not the least distinction between this macula and the other hypostoma, only, that it is free of pores.
Calymmene tuberculata Brünnich.
We have succeeded to prepare a horizontal section of the macula. This is oval, perfectly homogenous and without any structure proper, showing only indistinctly a mottled medley of pale, brownish spots in the clear mass. There is in both maculæ a small dark, angular spot placed in their inferior part, [fig. 9].
Centropleura Angelin.
This author has delineated most distinct maculæ in Centr. Steenstrupi [pl. III fig. 4, b]. two, small elongate darkish spots, near the posterior margin. I have not seen any specimen of this trilobite and have consequently not been able to follow up Angelin's observation.
Chasmops M'Coy.
In his work on the Russian Phacopidæ Friedr. Schmidt has given the figures of Chasmops Eichwaldi and Ch. Wesenbergensis showing, as it were, patches of black maculæ. But as there is no mention of them, and as they are quite unlike the macula? which are described here, I am in doubt of their real nature. We have figured a macula from the hypostoma of a Chasmops, probably Ch. macroura, from a drift block near Rostock. It is elongate and narrow, more than four times longer than broad. Its smooth surface is a little scooped out and no structure is observable.
Chirurus Beyrich.
We find the hypostomic maculæ annotated and delineated as occurring in the following species.
Ch. bimucronatus Salter pl. 5, fig. 5 as two narrow elongated pits, surface seems intact. No mention of them in the letter-press.
Ch. exsul Schmidt pl. VI fig. 9. Indications of the oblong, narrow maculæ.
Ch. macrophthalmus Schm. pl. VII, figs. 1 c, 2. Faint indications of narrow pits.
Ch. Quenstedti Barr. Tab. 42 fig. 3. Maculæ near lower border as two narrow slits.
Ch. spinulosus Schm. pl. VII fig. 10. The lateral pits well visible.
Ch. tumidus Schm. pl. VIII, fig. 22. The pits are short, but present.
The exterior surface of the hypostoma is entirely covered with small wartlets, closely set, of different size in the various species, and spread between them lie a great number of elongate smooth spots dispersed, on the interior side of the hypostoma showing an excavated surface ([pl. III fig. 16]).
Chirurus (Cyrtometopus) clavifrons Dalm.
The shape of the hypostoma is shown in [fig. 21 pl. III]. It is regularly shield-formed with posterior margin rounded. A shallow groove running parallel with the margins forms a large, faintly elevated border. The anterior margin projects on both sides in a short, backwards directed, broad and fiat wing, hollowed out with a pit on the front side, and there is a posterior lateral wing, midways between the anterior and posterior margin short, blunt, directed obliquely backwards. The whole exterior surface is covered with diminutive tubercles, the above mentioned smooth spots interspersed on the central disk. A little below the middle there are two lengthened pits, so shallow that they often are not discernible. In a specimen of Chirurus spinulosus Nieczkowski from Estland there are two, almost 3 mm. long dots, one on each side above the maculæ as the [fig. 21] shows and in several others of this generic group there are also indications of similar. I cannot compare these dots with anything more than the lengthened spots visible in Acidaspis crenata.
The maculæ are wanting in this species, but the cephalic eyes are well developed. A section of them [fig. 19] shows the enormous difference of the shell in the eye and the surroundings where the shell surpasses it many times in thickness. The eye consists in fact only of the ovate, beadlike lenses of which a string is seen sectioned in the figure mentioned. More enlarged ([fig. 20]) a nucleus is visible in each lens, and in a horizontal section ([fig. 18]) a little below the surface, where they are more pressed against themselves, they have a polygonal shape. The [figure 17] seems to represent lenses that have been much changed interiorly, having only a narrow zone left of the primary structure.
Chirurus glaber Angelin.
As a sample of the shape of the surface of the eyes in this genus, we have given a figure, showing the small rounded lenses.
Chirurus ornatus Dalman.
The cephalic eyes have globular lenses, of more than double the size of those in Chir. clavifrons. The difference between the lenses and the surrounding shell amounts at the highest to thrice the former, while in Ch. clavifrons it is at least seven times as much. In a horizontal section the lenses have a little, darkish irregular nucleus surrounded by a radiated structure ([fig. 12]).
The maculæ ([fig. 14]) are slightly concave, oblong, smooth, surrounded by comparatively large granulations.
Chirurus speciosus Dalm.
The exterior surface of the hypostoma is sparingly covered by granules of larger size than in other Chiruri. The maculæ as seen in a cast are prominent near the posterior border, sunk in their centre and surrounded by a narrow distinct border line. In a horizontal section it as not been possible to detect any peculiar structure, only a dark border surrounding the interior clear surface.
Chirurus spinulosus Nieczkowski.
We have had the interior surface of a hypostoma at our disposal from the lower Siluria, of Esthonia (Kuckers C2) belonging to Dr. G. Holm. The macula are well preserved, standing out black on the white surface, oblong, 1,5 millim. in greatest length ([fig. 15]). There are indications as of a great number of small lenses on these maculæ. Remarkable are the two elongated spots, almost 3 mm. in length, above these maculæ, of which already mention has been made under Ch. clavifrons.
Chirurus sp. indet.
The hypostoma of an undescribed species from Öland. The maculæ are lengthened, ellipsoid and smooth tubercles lying in a groove. The finely granulated surface of the hypostoma shows rare smooth spots interspersed, being on the inside smooth pits, from which again in casts of the hypostoma larger tubercles are moulded. Of the same nature are those that are visible on the nucleus of Chir. speciosus and others. There are still some undescribed Lower Silurian species, for instance one nearly related to Ch. conformis with tubercular maculæ. A large hypostoma from the Leptæna limestone of Dalecarlia, 33 millim. in length, has a longitudinal macula, 2 millim. in length, as seen on the inside of the fragment.
On [pl. VI fig. 10] a little hypostoma is delineated that shows some resemblance with that of the Chirurus, but probably belongs to some other, unknown genus. It is Upper Silurian, found at Mulde in Fröjel, Gotland. It is of an elongated ovate shape, with the anterior border faintly arched, the anterior wings broad, truncate. A narrow elevated border surrounds the lateral and posterior margins. The somewhat vaulted surface is covered by fine granulations, and the macula, situated on equal distance from the anterior and posterior margins are ovate, smooth and directed obliquely inwards. There are two minute pointed processes on each side of the lateral borders and one on each side of the pointed posterior margin.
Ctenopyge Linsn.
Ct. spectabilis Br. (I, pl. XII, fig. 12 a).
A small lengthened, apparently smooth hypostoma with two globular tubercular maculæ near the posterior border. No mention made of them in the description of Brögger. Cambrian. I am not, however, quite sure whether this really belongs to a Ctenopyge, as it rather more resembles the type of a Peltura as shown in Pelt. scarabæoides. As Ctenopyge is so nearly related to Sphærophthalmus it could be expected to see its hypostoma of the same type and the maculæ entirely wanting. The eyes of this genus have been described above at [page 29].
Cybele Lovén.
Cyb. bellatula Dalman.
We give a new figure of the hypostoma to complete that of Fr. Schmidt, pl. XIII fig. 9. The anterior wings are larger and obtuse, excavated below and the border rim is not so distinct all round as he has figured. There are two pair of oblique, lengthened grooves above each other. The opposite do not, however, join to form a coherent groove across the median field of the hypostoma, which is level between them. No maculæ. Nearer the anterior border close below the wings, there is a small roundish, dark spot. The surface of the hypostoma is granulated.
Cyphaspis Ang.
Cyph. elegantula Ang.
The hypostoma is elongated, anterior margin evenly arched, anterior wings triangular, lateral margins faintly curved as well as the posterior margin, at both sides of which there is one diminutive spine. The posterior wings are short forming a spiny process from the lateral margin. The surface of the hypostoma is smooth with a shallow transverse groove on the inferior moiety and two corresponding lateral impressions below it, one on each side. The elevated lateral borders are longitudinally striated by a few terrace lines. There are no maculæ visible. The eves consist f short hexagonal prisms ([figs. 22, 23]) nearly resembling those of Proetus.
Dalmanites Emmrich.
In this large genus the hypostoma has been figured in a great number of species. But it is only in the following that we, at least in the figures, if not in the descriptions, are able to detect the maculæ.
D. atavus Barr. Suppl. pl. 5 fig. 14.
D. Calypso J. Hall, vol. VII, pl. XI A. fig. 21 p. 66 »Postero-lateral pits, moderately strong and elongate». There are casts of the two well distinct maculæ.
D. caudatus Brünn.
a. Forbes and Salter in British Organic remains, Dec. II pl. 1, fig. 3 give a good figure, showing the oblique pitlike maculæ. On page 2 is told that »a pair of lateral strong indentations indicate a second furrow above» (»the transverse furrow between the tip»).
b. Salter, pl. 3, figs. 7, 8, bad figures, the description verbally the same as above. What Angelin pl. VIII, fig. 2 c gives as the hypostoma of »Ph. caudata» is in fact the hypostoma of a species of Lichas.
D. Mac Coyi Barr. Supl., pl. 13 f. 32. This magnificent hypostoma belongs to the same group as D. micrurus Hall as to which we may hesitate whether there not be a double pair of maculæ above each other near the posterior border. In this species, however, there is the transverse lower groove and above probably the two maculæ united by a groove so closely as to resemble the lower groove.
D. micrurus J. Hall, Pal. N. Y. III pl. 74 fig. 20. Shows, as it were, a double pair of maculæ, very like D. spinifer Barr.
D. rugosus Barr. pl. 24 f. 23. The two maculæ in the ordinary place, without being united by a groove.
D. socialis Barr. pl. 26 f. 21. Rather of an uncommon shape, not so triangular as in the other species, the two maculæ distinct as narrow slits. P. 553 »Vers l'extrémité du corps central, on aperçoit de chaque côté, près du bord, une impression oblique, alongée et arquée».
D. spinifer Barr. Pl. 25, fig. 20. A large beautifully preserved hypostoma, shows what I think we positively must interpret as a double pair of maculæ close above each other. As stated above there is reason to believe that in other species of Dalmanites there also are indications of four maculæ, though not so evident as in this species. Barrande says nothing about this remarkable feature.
Dalmanites imbricatulus Angelin.
The interesting image, as given by the agglomerated eyes, is represented in the magnified [figure 43 Plate III]. The lenses lie there separated from each other at much varying distance, some in close contact. The presence of the delicate covering membrane is clearly seen, being of white colour contrasting with the black, glossy lens. It is much lacerated and only preserved around the periphery. The size of the ocelli is on an average 0,5 mm., a little one has 0,3 mm. The surface between the ocelli is most finely granulated. The granules scarcely attaining the fourth of the granules in Dalm. vulgaris.
In a vertical section the regular biconvex lenses are seen to be covered with the extremely thin integument, which is a direct continuation from the test. The test between the lenses is perforated by some longitudinal tubes, as usual in the skeleton of the trilobites. We have not found any hypostoma with the maculæ.
Dalmanites obtusus Ldm.
We have not succeeded in finding any hypostoma, but as the structure of the cephalic eyes is sufficiently well preserved, we here describe it. The lenses seen from the surface look globular, and in a vertical section they are ovate and in a horizontal circular. They are covered by a very thin and delicate membrane, that envelops their superior moiety completely and between the eyes it is enclosed by the surrounding test and growing out from it. In the horizontal section, [fig. 45], it looks as a circular frame around the lenses and the membranes of two contiguous lenses sometimes lie so close, that they entirely fill up the interspace between the lenses, leaving only a narrow slit between them marking their boundaries. The space between the lenses, fig. 46, is porous, being perforated by tubes, which continue vertically down through the shell of the cheek.
Dalm. sclerops Dalman.
Commonly this species has been ranged with Phacops, but it is evident that it as to the conformation of its glabella and the head in general is highly discrepant, and in these respects is more concordant with Dalmanites, though still deviating as for instance in the pygidium and some details of the head. Therefore it may be justified with Friedr. Schmidt[41] to consider it as a generic division, though not, as he has it, as an independent genus, but as a subgenus of Dalmanites.
[41] Revision, I, p. 76.
The granulated hypostoma differs from that of Dalmanites proper in having the anterior margin strongly arched, the pointed wings almost on the median line of the hypostoma. There is one groove near the posterior border and the maculæ lie as two, narrow long, crescentic grooves close below the wings. The few sections give no clear idea of any structure.
The cephalic eyes are constructed on the same plan as in the previous species, the lenses being surrounded by a frame.
Dalm. vulgaris Salter.
The hypostoma has the common shield-like shape, [fig. 51], anteriorly slightly arcuated, the short obtuse wings curvated towards the interior surface. It is finely granulated. The groove does not go so near the cuspidate posterior margin as in other species. The two maculæ, [fig. 52], are narrow and elliptic, somewhat convex and lie in a little concavity, a narrow smooth space around is devoid of the granules, which cover the surface of the rest. They show absolutely no structure, excepting a few indistinct blackish spots.
The cephalic eyes have been often described and delineated. Most detailed are the descriptions and figures given by Salter in the Memoirs of the Geological Survey Dec. II. pl. 1. In the figure 4 a part of the surface is shown in well preserved state. He says that »the cornea is ... present and distinctly convex over each lens, the intermediate portions being ornamented with tubercles and granules». He thinks also that there are undeveloped or small lenses between the larger. His fig. 5 represents the frames around the lenses, these being probably lost. His explanation of fig. 6 I cannot understand. There are weathered or spoilt lenses figured.
There is no doubt that the granulated surface of the head continues between the more or less free lying lenses as shown here in a new figure taken from a specimen from Dudley, [fig. 47]. The granules of the surface are larger and more rare between the ocelli than for instance in Dalm. imbricatulus. Small lenses, undeveloped as Salter calls them, may in fact only be such granules of somewhat larger size than usual. Lenses having the appearance as in Salter's fig. 6, as if a covering was partially destroyed and the lens visible below it, I have also found in Swedish specimens, as represented in [fig. 47]. In a vertical section, [fig. 50], a delicate covering integument is seen and the oval lenses lie regularly with their frames around them quite as in Dalmanites obtusus and in horizontal sections, [figs. 48, 49], there are also the same sort of black, irregular dots, being the sectioned tubes of the surface between the lenses. But in another feature there is great interest. Beneath each lens there are fascicules of tiny rods, twice as long as the lenses. They are represented in a vertical section in [fig. 50] and in a tangential section in [fig. 49].
As our imperfect knowledge of so delicate anatomical structures in fossil crustaceans does not admit of secure comparisons with the visual organs of recent crustaceans, no suggestion can be given to interpret their nature. Probably they have no connection with the structure of the eyes and it is not even certain that they are of organic origin.
Dysplanus Burm.
If there were no other characters to distinguish this genus from Illænus its strangely deviating hypostoma must do it. We have examined two species and in both the hypostoma is almost oval, evenly rounded both anteriorly and posteriorly. Near the anterior border two narrow, long hornlike wings project. They are flattened, thin and lamellar from the basis and end in a hollow, acuminated point.
D. centrotus Dalm.
This species has the maculæ placed near the posterior border, in a semilunar flat, devoid of the terrace-lines. The maculæ are elongated tubercles and do not show any structure when sectioned. The cephalic eyes have a thin membrane covering the short cupshaped lenses, which, closely packed, in the transverse section give the usual polygonal mosaic of hexaeders. They have a radiate structure, when decayed.
D. ladogensis Holm.
A specimen from Öland has the maculæ lying in the large field of terrace lines above the smooth zone at the posterior margin of the hypostoma. They are larger than in the former species, pearshaped, smooth and no structure visible.
Encrinurus Emmr.
I cannot find any former figure indicating the maculæ of the hypostoma in this genus than the only one of Angelin, Pal. Scand. pl. IV fig. 6.
As to the shape of the singularly formed hypostoma in the three Swedish species Encr. punctatus, obtusus and lævis we have found the anterior border so characteristic in them that it is sufficient to distinguish them from each other and to annul the doubts concerning the distinction of Encr. lævis from Encr. punctatus. We therefore in [pl. IV figs. 12-17] give the anterior margins of these three species, along with the profiles.
Encrin. punctatus Wahlenberg.
The surface of the hypostoma is so finely and obscurely granulated that it seems to be almost smooth. It is in the shallow groove above the lamellar tongueshaped posterior border that the elongated tubercular macula are situated. They are prominent and in horizontal sections they have a surface mottled by irregular black and white specks, the whole surrounded by a white border, [fig. 6]. This border and the other surface is pierced by series of parallel slitlike pores, [fig. 7], a feature peculiar to this genus. The vertical sections indicate a chain of indistinct prisms with a black, rounded central hollow between thick strata of clear whitish shell substance, [figs. 8, 9]. The shell substance on the sides is pierced by the straight, black tubes.
The cephalic eyes have the prisms indistinctly developed or badly preserved with a diameter of 0,04 millim. and height of 0,07 millim. In Encr. lævis the prisms are more distinct and larger with a diameter of 0,00 millim. ([pl. IV figs. 10-11]).
The maculæ are well developed, ovate, in E. lævis and obtusus and in the same position as in E. punctatus.
Griffithides Portlock.
The hypostoma of Griff. globiceps Phill. shows according to the figure given in H. Woodward's work pl. VI fig. 5 two somewhat obscure maculæ, but no mention is made of them in the description.
Harpes Goldf.
Harpes d'Orbignyanus Barr. Novák II Taf. I fig. 4, with small black maculæ. Harpes venulosus Corda. Novák II, Taf. I figs. 1, 2. Two narrow, horizontally placed maculæ.
Harpina Barr.
H. prima Barr. Novák II Taf. I fig. 5. Two maculæ in continuation of a narrow groove, at the base of the central globosity.
Holmia Walcott.
Holmia Lundgreni Moberg, Sveriges äldsta trilobiter, Tafl. 14, fig. 10, 11, 12. The maculæ are much prominent and tubercular, elongate narrow, oblique but not mentioned in the description. Hypostoma probably belonging to the terrace-line group. As well seen in original specimens and in the figures in Dr. Holm's paper on Olenellus (Holmia) Kjerulfi there are also in this species two distinct macula; near the posterior margin of the hypostoma, above the marginal groove.
Homalonotus König.
Maculæ have been previously observed in the following species.
Hom. delphinocephalus Green, Hall Pal. N. York vol. II pl. 68 fig. 11, incomplete figure with two well marked tubercular maculæ. No description.
Hom. Knighti König. Salter II, pl. 12 fig. 10. Maculæ well expressed as oblique tubercles. On page 120 it is said »with a pair of lateral tubercles well developed (as in Asaphus ...)».
This species is identical with Angelin's Homalon. rhinotropis, Pal. Sc., pl. XX fig. 1 e and his figure has two hollow maculæ seen from the inside of the hypostoma.
Homalonotus Knighti König (= Homal. rhinotropis Angelin).
The maculæ lie as oblong, smooth tubercles surrounded by the irregularly grown granules of the surface, just below the central globosity of the hypostoma. In a preparation from the interior surface of the macula, as seen in transmitted light, [fig. 21], this is white with only a few grey spots and on the superior border black streaks, lying obliquely. The substance of the shell around the macula is perforated by the minutest pores amongst which a few larger are intermingled. The granules are also perforated as those in Calymmene and the tubes continue through the shell, visible by their black colour.
Hysterolenus Moberg.
Hyst. Törnquisti Mbg., En trilobit från Dictyograptusskiffern p. 320. In the descriptive letter-press, there is said, that in the exterior (lateral) parts of the anterior groove of the hypostoma a distinct tubercle is to be seen on each side. But none of the figures given, Pl. 17 figs. 6, 7, shows them or at least very indistinctly.
Illænus Dalman.
It is remarkable, considering the great number of species in this almost exclusively Lower Silurian species, that there is none but a single species in which the maculæ hitherto have been delineated. This is Illænus angustifrons var. depresses Holm I, pl. VIII fig. 18. A little, fragmentary hypostoma with globular maculæ.
Illænus Chiron Holm.
The hypostoma has the shape characteristic of all true Illæni, viz. a straight anterior margin with two large flat rectangular or nearly quadratic wings, a globular or spherical median field, forming the chief portion of the exterior surface, which is covered by some rare, fine, terrace lines. In some other Illæni the surface is smooth. At the posterior base of the median field the two maculæ are placed, tiny, oblong, but lying on the same level, at right angles to the median longitudinal axis of the hypostoma. They are covered with lenses of irregular and indistinct shape [fig. 25]. I do not think that they in this and the other species of Tiberius are lenses of the same conformation as in Bronteus, they are rather the tops of the subjacent lenses which are of the prismatic form. The cephalic eyes do not show any lenses on the outside. They have a narrow, opaque exterior integument, hiding hexaedral, thickly packed, straight prisms, forming a stratum in thickness of 0,2 millim.
The hypostoma consists of several thin layers of superposed calcite, which easily peal off, so that it is seldom that the true exterior sculptured surface remains.
Illænus Esmarki Schloth.
This the most common of all the Illæni has in several specimens shown the hypostoma with the maculæ in situ. The hypostoma is smooth excepting on the marginal ridges where there are some terrace lines. On the median axis of the central gibbosity ([fig. 30]) five faintly visible transverse folds are situated, a curious feature which we have not observed in any other trilobite. The maculæ are exceedingly small, [fig. 31], elongate, spindleshaped and acuminated in both ends. On their surface there are accumulated several lenslike globules. As the horizontal sections show there are, however, no regular lenses, but a network of polygonal meshes with radii in the hollows, [fig. 32]. The perfect identity of this structure with that of certain states of the cephalic eyes will at once be perceived by comparing this figure with [figure 26] representing the same sort of polyedric meshes, though a little more regular in the cephalic eye, where, however, the irregularity also prevails near the periphery of the eye. If we now compare the sections of the cephalic eye of Illænus Chiron and the section of the regular and nearly unchanged eyes of Ill. Esmarki with the second section of the latter, we cannot avoid the conclusion that we in those of Ill. Chiron, [figs. 22, 23], and the first of Ill. Esmarki, [fig. 27], see the primitive and intact state of the lenses and in the last sections of Ill. Esmarki, [figs. 26, 28], the changed and deteriorated state of the same prismatic lenses.
The vertical section of the hypostomic eyes [fig. 33] shows in their present changed condition an evident longitudinal direction of the chief elements, quite as the lenses of the eyes. But in how far there ever has existed such prismatic lenses, now changed, we have no evidence definitely to ascertain, but the probability seems to be great that this was the case.
Illænus gigas Holm.
The maculæ in this as well as the other Illæni are so faintly elevated and so inconspicuous that the greatest attention is needed to find them.
The maculæ are elongated, narrow, elliptic on the same level and parallel with the straight anterior margin of the hypostoma. A cast of the left macula is pitted by numerous marks of the lenticular globules of the surface, [fig. 37]. In the cephalic eye the prisms are seen translucent beneath the cornea. Near the superior border they are larger, [fig. 34], almost the double size of those near the inferior border, [fig. 35].
Illænus sphæricus Holm.
Comes near to the previous species. The globules on the surface of the elliptic maculæ are not, however, spread over the entire macula, leaving only a narrow border all around free.
The lenses in the cephalic eyes [fig. 42] are globular as seen on the surface, alike those of the maculæ.
Illænus Roemeri Vollborth.
The maculæ lie on elevated tubercles and are of a strange shape, being sharpely pointed outwards, rounded inwards. That in [fig. 39] is still more peculiar with a narrow stripe on the interior and upper margin. The lenses are larger and more distinct than in any other species of this genus.
Beside the species now mentioned we have also observed hypostomas with maculæ in Illænus laticlavius Eichwald from Estland, in Illænus Linnarssoni Holm, and in an unnamed species from Dalecarlia figured without macula by Dr. Holm in his memoir on the Illænidæ pl. III fig. 22. We have not seen any lenses on the maculæ of these three lenses.
Lichas Dalman.
We have not been able to find any notice about the exceptional maculæ of this genus. There is only a figure of the hypostoma of Lichas hylæus Hall Pal. of N. York vol. VII pl. XXV fig. 5 which would seem to show a pair of narrow, oblique maculæ, but these are quite without resemblance with the maculæ of the true Lichas.
The hypostoma of Lichas is of a characteristic type, broad, with the incised posterior margin and its two lappets reminding of Asaphus, with its large rounded, faintly elevated median part near the anterior margin reminding of Illænus, as also do the lenticles of the maculæ. The maculæ are of a minimal size, scarcely 1 millim. in diameter and thence easily overlooked. [Pl. IV figs. 46, 48.]
As the hypostomas in this genus generally are found detached it is often difficult to decide to which species they have belonged. So it is with that delineated in [Pl. IV] fig. 44 natural size. The two maculæ are situated near the centre of the hypostoma, just below the grooves which surround the large, globular disc. They are sunk in a little cavity, rounded and entirely covered by globular lenses ([pl. IV fig. 46]).
In another detached hypostoma of an unknown species the macula is smaller and the few lenticular globules larger than in the other, [fig. 48].
Lichas latifrons Angelin.
The maculæ, seen in a thin section from the inside, are reticulate, [fig. 49], or of a pattern exactly like that in the changed eyes or maculæ of Illænus and even Asaphus, and thus indicating that it was composed of short prismatic lenses. The vertical section of a macula of a specimen from another locality rather suggests the presence of large sphærical lenses than elongated prisms. The great thinness of the macula in contrast to the excessive thickness of the shell around the eye is seldom so evident as here.
A specimen of an unknown species, found detached, has the macula? unlike the other species, oblique with the inferior end directed inwards, placed on the top of a sort of a little elevation, [fig. 47].
Megalaspides Brögger.
This author has a figure of M. dalecarlicus in II pl. 1 fig. 19 and also of an unnamed species fig. 20, both showing globular maculæ. In Holm's original description of his Megalaspis dalecarlicus (Trilobiten des Phyllograptusschiefers Dalecarliens figs. 8, 9) the same hypostoma is also figured. This genus seems to have a closer affinity with Ptychopyge than with Megalaspis.
Megalaspis Ang.
The following figures of hypostomas with maculæ have been published.
Megal. limbata by Brögger I, tab. XII, fig. 10, and in II pl. 2, fig. 22, somewhat differing from the first figure.
Meg. planilimbata Brögger II tab. 2 figs. 21, 21 a. Both figures incomplete and incorrect.
The hypostoma has a large central ovate gibbosity with the maculæ near its basis, surrounded by the winglike expanded limbus.
Megal. attenuata Wahlenberg.
The long and narrow maculæ are crescent-shaped and project beyond the hypostoma supported on the superior surface of a sharp extenuated edge where they rest as on a shelf. They consist, as seen in vertical sections, [figs. 5, 6], of alternating rows of light and black streaks, and in a horizontal section the spongious network appears, as it is found in Asaphus and others and thus indicating the tendency of this organ to assume a prismatic structure though undeveloped.
The cephalic eyes in M. attenuata form the most regular hexaedral prisms [fig. 1 a], short and covered by a thin cornea. The border zone of the eyes has a structure exactly like that of the maculæ, the same as in the spongious zone of Asaphus.
Megal. limbata Boeck.
We give a new figure of Brögger's original specimens, as his figure 23, Tab. II is incomplete and incorrect. The maculæ are situated a little higher up on the sides of the central gibbosity.
Megal. planilimbata Ang.
The maculæ are crescent-shaped and not so prominent as in Meg. attenuata and the visual spot in the same position as there.
To observe is that the general structure of the hypostomic shell in this and many other species is prismatic.
Nileus Dalman.
Hypostoma with maculæ figured in N. armadillo by Angelin pl. XVI fig. 5 c, and Br. II pl. 3 fig. 40.
The hypostoma in this genus is transversal with large lateral expansions, the central convexity only faintly elevated and almost coherent with the posterior part. The maculæ lie exactly on the horizontal median line of the hypostoma. The anterior wings are well developed like slightly bent acuminated horns and the posterior wings are conical points. The terrace lines are transverse and a little wavy.
Nileus armadillo Dalm.
There is some variability in the hypostoma of specimens from different localities. The most known and numerous specimens are from the renowned localities of Östergötland. They are decidedly transverse, the proportion of the hight to breadth is as 3 to 5 and the narrow border runs all round. In specimens from Kongslena, Vestergötland, the proportion is as 15 to 22, in specimens from Sandvik, Öland, again, we have 13 to 23 or 6 to 11. The maculæ, [fig. 14], are oblong, inwardly pointed, quite smooth, with a little concavity in their centre, and their surface is slanting obliquely towards the surface of the hypostoma, almost as in Niobe. There is no trace of any structure.
The cephalic eyes are covered by a comparatively thick membrane, a direct continuation of the general test, [fig. 12], hiding a row of prismatic lenses. In some specimens elongate, clear, crystalline rods continue downwards from the bases of the lenses and are probably nothing but inorganic crystals of calcareous spar, [fig. 11].—The eyes are larger comparatively than in other species.
Nileus (Symphysurus) palpebrosus Dalman.
As to its general form the hypostoma resembles that of the preceding species, but there is no border line, which is so characteristic to the former. The maculæ have the same form and are likewise situated on the median line. The cephalic eyes have prismatic lenses with much convex bases, in a transverse section they are hexaedral and of internal radiate structure.
Niobe Angelin.
The following figures have formerly been given of its hypostoma with maculæ.
N. emarginula Br. II tab. 2 fig. 33.
N. explanata Br. II tab. 2 fig. 35.
N. frontalis Br. II fig. 37.
N. insignis Br. I tab. IV fig. 1 d, 11 tab. 2 fig. 28.
N. læviceps Br. II pl. 2 fig. 34.
The maculæ are distinguished from all other sorts of maculæ, excepting those of Megalaspis, in being, as it were, shelved on a sort of support, formed through the bulging of the hypostomic surface just below them, one such console for each. Their surface lies consequently almost rectangularly to the adjoining surface of the hypostoma. They are white and smooth.
Niobe frontalis Ang.
The pocketlike consoles are shorter and broader than in N. læviceps and the maculæ also larger and of an elliptic shape. In a horizontal section near the surface the macula has a speckled appearance of black dots, indicating an undeveloped prismatic structure, much more primitive than in the Asaphidæ.
Niobe læviceps Ang.
It differs from the preceding through longer consoles below the maculæ, which are oval and in a horizontal section exhibit the same spongious structure, [pl. V fig. 25]. The cephalic eyes consist of polyedric prisms, which through deterioration show a central cylindrical core surrounded by darkish matter like Asaphus. The passage from solid, homogenous prisms to changed ones is most evident in the [figure 22].
Ogygia Brongn.
Figures of the maculæ are found in
O. Buchi Salter, Mem. Geol. Surv., Dec. II pl. VI fig. 3 p. 2, where it is stated that »there are two transverse furrows near the apex, with compressed tubercles between them» There are two narrow crescentic maculæ and between them a little lower a longer, crescentic ridge, which may be the ridge that in other species connects the maculæ. Moreover, the shape of the hypostoma is not concordant with that of the other species. In Monogr. Brit Trilob. pl. 15 figs. 2, 3 there is only a reproduction of the former figure.
O. corndensis Murch. Salter in Monogr. pl. 16 fig. 10 interior side of the hypostoma with two lateral maculæ. In the description two pairs of furrows the uppermost is the maculæ. H. Wyatt-Edgell »On the Genera of Trilobites Asaphus and Ogygia and the Subgenus Ptychopyge» in Geol. Magaz. 1867, p. 14, 15 fig. 2 probably a cast with two oblique impressions of maculæ.
O. dilatata (Asaphus) Sars var. Sarsi Ang. Sars in Isis 1835 p. 342, pl. IX fig. 11. The figure has two small maculæ near the posterior margin and the author says »mit einer starken Vertiefung oder Einschnitt an jeder Seite des Endes». Angelin pl. XLII fig. 1 b. Brögger's fig. 38 pl. 3, in II, collies near to the specimen, which is described below, also belonging to Dr. G. Holm.
O. dilatata var. Sarsi Ang.
The hypostoma is in so far deviating that the terrace lines have a nearly vertical direction and in this respect much resemble those of the Phillipsiæ and the Proeti. The maculæ, near the exterior angles of the border groove are pear- or spoon-shaped with the apex directed downwards and outwards, and in consequence the longitudinal axis quite opposite to that of the usual direction. The surface is smooth, a little concave. No structure has been discovered. From the pointed apex a slightly elevated ridge runs along the posterior border of the hypostoma and joins the apex of the other macula.
Paradoxides Brongn.
The oldest record of any hypostoma at all amongst the trilobites is that given by Wahlenberg in his Petref. Suecana (1818) p. 37 Tab. 1, fig. 6, which he, however, considered as the head of the trilobite which he named Entomostracites bucephalus. Figures with maculæ have been given of
Par. Davidis Salter Mem. Geol. Survey Dec. XI pl. X, fig. 3, a fine figure of a hypostoma with two large oblique maculæ. Linnarsson in »de undre Paradoxideslagren vid Andrarum» pl. II fig. 2 delineates a gigantic hypostoma with two crescentic tubercular maculæ, having 10 mms. in length.
Par. Forchhammeri Angelin Pl. II fig. 3. Brögger, Paradoxidesskiffr. vid Krækling tab. II fig. 10 two marks somewhat different from Angelin's figure. Linnarsson 1. c. pl. I figs. 9, 10 two hypostomas with maculæ.
Par. Tessini, var. Wahlenbergi Angel. Pl. I a, fig. 1 b.
Par. Tessini, var. oelandicus Ang. Tab. I a, fig. 2 b. Angelin's figure of this hypostoma is incomplete in so far that he had not found the peculiar falciform horns on both sides of the posterior border. Pl. V, fig. 33.
Par. rugulosus Corda has, according to Brögger, Krækling, pl. II fig. 2 two long, straight callosities, probably corresponding to the maculæ in other Paradoxidæ.
Chr. Boeck gives in Mag. for Naturvidenskaberne Bd 8, 1828, in his paper on the Trilobites on the plate fig. 16 the hypostoma of a Bohemian species with well developed concave maculæ, probably a cast. He compares it with the Entom. bucephalus of Wahlenberg, but says he cannot explain its nature.
The maculæ of P. oelandicus, pl. V, fig. 34, are tubercles, oblong, smooth, and have invariably an oblong scar along the centre, where probably once a thinner membrane, as in the cephalic eyes of Calymmene, contained the lenses.
Peltura M. Edw.
Peltura scarabæoides Wahlenb.
We have given a figure of the hypostoma, showing two large, elongated tubercular maculæ or rather resemblances of such, as they are covered with terrace lines as well as the rest of the surface. They have, however, the same position as real maculæ might have, but are directed in a line parallel with the longitudinal axis of the hypostoma. The cephalic eyes have been described above at [page 29].
Phacops Emmrich.
Maculæ figured of Phac. (Acaste) Downingiæ. Salter Monogr. pl. 2 fig. 34 b, p. 25 »high up on each side a small tubercle».
Phac. rana Hall Pal. N. Y. vol. VII, pl. VIII A, fig. 18. Hypostoma with the large commalike maculæ high up.
Of the following species Ph. cephalotes and Ph. macrophthalmus belong to the genus Phacops proper, while Phac. quadrilineata, as well as Ph. Downingiæ are of a quite different generic type. The name Acaste proposed for them is not good as Leach in 1817 named a Cirrhiped as Acasta.
Ph. cephalotes Barr.
The hypostoma is elongate, almost triangular, tricuspidate at the posterior border, a little above this there is a crescent-shaped ridge, and still a little higher above this and the median horizontal line, the two macula, as diminutive tubercles. Seen with magnifying powers they are slightly oblong or reniform.
Ph. macrophthalmus Burm.
The triangular hypostoma has a little below the median horizontal diameter two shallow grooves, and a little above the same diameter the extraordinary minute maculæ may with some care be observed, [figs. 8, 9]. They are visible as a small bare, smooth spot amongst the curious elevated and incoherent terrace lines of the surface. The exceedingly small size of these maculæ naturally does not allow any research of their structure. They are rather in this species and many of the congeneric to be regarded as small ocelli. In some specimens these spots are quite rudimentary and are reduced to the smallest area possible. The anterior wings are of a peculiar earlike shape, finely striate, enlarged and flat near the hypostoma and ending in a little hollow horn ([figs. 6, 7]). A little below it, the posterior wing emerges as a small angular knob. The aggregate cephalic eyes are more distinct.
As the vertical section learn us the regular ovate lenses consist generally of clear, crystalline calcareous spar and are on the surface covered by an extremely thin membrane, not a common one for all, but a separate cornea for each lens, which envelopes its superior moiety and continues down as a thickened appendix between the lens and the interstitial test ([figs. 3-5]). In the horizontal sections it surrounds the lenses as an annular wall. Around some lenses there is as if still another ring lay outside this, but more indistinct, probably only clearer portions of the test surrounding them. The cheeks close to the lenses are indistinctly perforated by longitudinal tubes.
Phacops (Acaste) quadrilineata Angelin.
We have not succeeded in finding any well preserved hypostoma.
The lenses of the cephalic eyes resemble much those of the preceding species though more elongate and ovate. They are covered by a membrane which surrounds their upper moiety and between the lenses joins the interstitial test. In the horizontal section these membranes surround the lenses as an annular wall.
Phaëtonides Barr.
Ph. Stokesi Murch.
The hypostoma somewhat reminds of that of Calymmene, has projecting angles and a prominent knob below the straight anterior border. The two oblique and oblong macula are tubercular and placed near the lateral borders. This hypostoma is upon the whole much related to that of Phillipsia and Proetus.
Phillipsia Portlock.
De Koninck figured a hypostoma of Phillipsia in his »Description des Animaux Foss. de Belgique» as Cyclus Brongniartianus pl. LII fig. J and Novák in III figs. 6, 7 copied it and corrected the error. It is provided with two globular maculæ united by a curved ridge.
Phillipsia Eichwaldi Woodw. Novák III fig. 5 a hypostoma with globular maculæ. We have been able to examine three different, unnamed or undeterminated species, which we distinguish by numbering them.
Phill. No. 1 from the Keokuk group of Crawfordsville, N. America. The hypostoma is broader than in the other species. The nearly horizontal elongated maculæ placed near the median line, close to the lateral margins.
Phill. No. 2 ([pl. VI, figs. 15-18]) probably from the Carboniferous formation of Belgium is of a lengthened form. The elliptic maculæ in the groove, below the central elevation. They are smooth and surrounded by a flat border.
The cephalic eyes are much destroyed, but seem to have consisted of somewhat semi-prismatic lenses or prisms of the same type as Dysplanus.
Phillipsia No. 3 ([pl. VI figs. 12-14]) from the Carboniferous strata of Beeren Eiland communicated by Hr J. G. Andersson. It comes near the preceding, is lengthened, but has broader and shorter anterior wings and the inferior surface just above the posterior border peculiarly pitted. The macula lying above this pitted field are ovate. The lenses of the cephalic eye are hemispheric on the surface.
Platymetopus Angelin.
Pl. planifrons Angelin.
This genus may well be kept distinguished from Lichas in consequence of its peculiar hypostoma and its still more peculiar maculæ, not to remind of the other deviating features. The maculæ lie at the end of narrow, sigmoid grooves of their own above the long transverse groove, separating the posterior border from the anterior elevated moiety. They are elongated, curved and entirely covered with a great number of irregular lenticles, translucent through the thin covering membrane.
Pliomera Ang.
Pliomera Törnquisti Holm in Trilobit. Dalecarliens p. 5 fig. 2.
While Pl. Fischeri and Pl. actinura not have any hypostomic maculæ, this species, which Dr. Holm with some doubt joins in the same genus, has a hypostoma, which as to its general shape resembles that of both the mentioned species, evenly rounded at its posterior border, with a shallow groove encircling the central field. A little below the horizontal diameter of this the maculæ stretch obliquely inwards and downwards. They rest on a little elevation and are elongate, anteriorly acuminated. The figure on Holm's plate does not show the maculæ.
Proetus Steininger.
There are some indistinct indications of maculæ in several of the hypostomas figured in Barrande's Système pl. 15.
Proet. signatus Lindstr., Gotl. Trilob. Pl. XV fig. 17, shows very distinctly the two maculæ.
Proet. concinnus Dalm.
In this, as well as in the few other species of Proetus which we have studied, there is that peculiarity of the surface of the hypostoma that two more or less distinct ridges form an acute angle just below the faintly arcuated anterior border. The terrace lines are exactly alike those of the Phillipsiæ, only visible on the central field and ceasing just above the maculæ, where there is a transverse smooth field. The posterior margin has two pair of very short points, one on each side. The macula are lengthened tubercles, somewhat bent and have an oblique position. They are surrounded by tiny prickles and as seen in [fig. 23] the macula proper forms a white, elliptic spot on the tubercle. The cephalic eyes are lengthened prisms covered by a thick membrane. They are convex at their base and in a transverse section they show a radiate structure of their interior.
Proet. conspersus Angelin.
We have figured a decorticated hypostoma with impressions of two reniform maculæ. The vertical sections of the cephalic eyes show a thin stratum where the lenses are much shorter than in the previous species and covered by a thin film of the common test. The same is also the case in a species which probably is Proet. verrucosus.
Proet. signatus Lindström.
On the lateral borders of the hypostoma there are short spines, two on each side, but they are placed differently on the specimens and even wanting in some. The elongated, elliptical maculæ bear in their lower end a white spot on which is seen a little cluster of a varying number of small, segregated ocelli, from three to five, according to the different specimens, like so many black points. The maculæ attain the exceptional length of 4,7 millim. in some specimens, but commonly only 2 millim. We have as yet not been able to detect these remarkable ocelli in any other species of Proetus.
Ptychoparia Corda.
Walcott in »Palæontology of the Eureka District», gives on plate X fig. 21 a representation of a hypostoma which near the posterior margin shows two maculæ as narrow, crescentlike ridges and above them two longer and thicker ridges directed obliquely toward the central axis of the hypostoma.
Ptychopyge Ang.
Previous figures of hypostomas with maculæ are the following:
Pt. aciculata Br. II, pl. 1 fig. 13.
Pt. glabrata Br. II, pl. 1 fig. 14.
Pt. aciculata Ang.
The broad hypostoma bears the elongated, acuminated maculæ in an oblique direction, contrary to what is seen in most other species. The anterior end slopes inwards and the lower end outwards, the surface, which in the cast drawn is surrounded by a marginal line, is directed sidewards against the broad lateral wings and is obscured by the declivity formed by the groove. The same disposition of the mature also occurs in Ptych. glabrata. These two species consequently in this respect differ essentially from the other four species which we have examined, but there is for the rest no reason to make these form different genera or subgenera.
Pt. angustifrons Dalm.
There are two varieties of hypostomas: one broad [fig. 32] and one more elongated [fig. 33]. The maculæ having a convex surface lie at the inferior end of the lateral grooves prominent on a semicylindrical support, slightly inclining outwards. In the broad variety they are oblique ([fig. 34]) and in the elongated variety they are horizontal ([fig. 35]). In a vertical section [fig. 36] we have the same indications of a prismatic arrangement in the maculæ as in the Asaphi. The horizontal section does not represent so clear and distinct a reticulate structure as in the Asaphi, the prisms are much smaller.
The cephalic eyes are composed of closely packed hexaedral prisms, the gradual change from solid homogenous ones to those which are interiorly destroyed, only with the exterior form preserved is easily seen. The figure of a horizontal section ([fig. 31]) gives a good idea of the peculiar destruction of the interior of these prisms. In the vertical section the prisms are not discernible.
Pt. cincta Brögger? according to Friedr. Schmidt in Mus. Holm.
The hypostoma which Brögger figures (II, pl. I fig. 2 a) as belonging to this species is completely identical with that of Pt. angustifrons, while the specimens which Fr. Schmidt has marked out as being the true Pt. cincta have a different hypostoma. The maculæ vary in the specimens, being in some placed horizontally or in a right angle to the longitudinal axis of the hypostoma, in others, again, a little obliquely outwards. They form prominent, free lying tubercles. It is from the superior face that the rudimentary lenses have left their traces, as seen in a longitudinal section ([fig. 39]). It is consequently this superior surface which is the macula proper.
Pt. glabrata Ang.
We have given a figure of the elliptic acuminate maculæ as seen on a cast of the inside of a hypostoma, the same which is the original to fig. 14 in Br. II.
Schmidtia Marcou.
Moberg has in his paper on »Sveriges äldsta Trilobiter», pl. 15, fig. 6, drawn a hypostoma of Schmidtia Torelli with scars of maculæ, placed like that of the Paradoxidæ. There is no mention made of them in the descriptions.
Trochurus Beyrich.
Tr. pusillus Ang.
The strange transverse, bipartite hypostoma carries the small, round, tuberculose maculæ on the superior moiety. As seen from the interior ([fig. 53]) they are obscurely maculate, but it is doubtful whether these black dots are derived from lenses, as the surrounding hypostomic surface also has the same mottled aspect.
Trochurus sp.
This hypostoma belongs to an unknown species of Trochurus from the Southern Gotland (Burs). Its macula are placed almost as in the preceding species, but are much larger. They are surrounded by prominent margins. No lenses have been discovered on them.