Researches on the Visual Organs of the Trilobites

by Gustaf Lindström

Blind Trilobites.

In the chaos of generic forms and in the great disagreement which prevails as to the systematizing of the trilobites of the Cambrian time, there is a thorough revision of them highly needed by a person having access not only to the literature, but also to the original specimens. It is almost impossible in the present state of things to tell with any degree of certainty how many well established genera had been living during that period. Hence the difficulty of fixing the systematic names of many specimens the visual organs of which are to be described.

My researches on the visual organs of the Cambrian trilobites are founded on the Angelinian Collection in the Swedish State Museum together with collections of foreign species, but also largely on the waste European and American literature, though we have to deplore the often occurring inexactitude of the figures, especially in the older works, and constructed or schematized figures in some of the newer ones, which give a quite false notion of the structure. There is no lack of figures to show how it ought to be, according to preconceived notions and, on the other hand, a great scarcity of representations, to show how it really is. In spite of all this there is a sufficiently great number of well established facts to demonstrate the organization of the Cambrian genera.

The trilobites of this division may be called blind only in so far as they have no eyes on the upper surface of the head, but they may have been provided with visual organs, though more imperfect, on the hypostoma as really seems to have been the case with some of them.

According to the structure of the head shield the blind trilobites may be subdivided into the following well characterized groups. These are:

I. Without facial ridge:
1) The Archæan Trilobites.
II. With facial ridge:
1) The Olenellidæ.
2) The Olenidæ and related.

I. Blind trilobites without facial ridge (= »eye lobe»).

Group 1. The Archæan Trilobites.

In these the head shield is in one piece without any facial suture and facial ridge, and without the least trace of anything that might be called a visual organ and they must consequently be considered as totally blind. In contradistinction to the following groups, excepting the oldest Olenellidæ, the head consists of only three parts, 1) glabella, and 2-3) the two fixed cheeks. These genera range from the oldest zone in which hitherto trilobites have been found, that of Olenellus (Holmia) Kjerulfi, to the zone of Paradoxides Forchhammeri, and some, as Agnostus, even continue as high in the Lower Silurian series as in the Brachiopod schists. Beside Agnostus the other genera are Conocoryphe (seven species in the Swedish Cambrium), Toxotis, Ctenocephalus (?), Elyx, Aneuacanthus, Conophrys and Microdiscus.

»Harpides» breviceps Ang., also belongs here. Anopocare of Angelin should also be regarded as one of this group. But it cannot be retained any longer because it is founded on two other, well known forms, being, according to Linnarsson, Peltura scarabæoides (pl. 27 fig. 1, a in Pal. Scand.) and young specimens of Sphærophthalmus alatus (ibid. figs. 1 & 2).

It is remarkable that some of the Conocoryphidæ have an imperfect facial ridge, to be compared with the commenced one in Sao Barr. I (pl. 7 fig. 9). So the American Con. trilineata and reticulate. Walcott U. S. Geol. Survey 10th Rep. pt. I, pl. XCV f. 5 & 6. It is, as it were, arrested in the development and these adult trilobites had stopped, where the larva of Sao was proceeding in its second stage. They are the forerunners of the blind trilobites with facial suture, belonging to the third group. It needs scarcely be mentioned that the genera now enumerated have hardly anything in common, beside the general character of the head, and that real affinity exists only between Agnostus and Microdiscus, and probably also between Conocoryphe, Ctenocephalus and Elyx.

Beecher[2] asserts that there is a suture in Agnostus, but in vain we have searched for it in numerous well preserved specimens and Dr Holm also denies its presence. Nor are there any signs of closed up sutures, which also could not possibly be expected in so early a stage of evolution. It may then be taken as well settled that a fundamental character in these the oldest[3] of all known trilobites is the total want of a facial suture and a compactness of the whole head shield which later is broken up in several parts through the disjunction of the free cheeks. In the Lower Silurian formation there are a few genera sharing in the same structure of the head shield, though by no means else related. Such are Dindymene, Areia, Carmon and in the U. Silurian Cromus. The two species forming Barrande's genus Dindymene are so dissimilar that Dind. Friderici Augusti had better to be removed to a new genus and the first described one to be retained as type of the genus Dindymene. The same is the case with Carmon, where the type species C. mutilus is blind and without free cheeks while the other species, known only by its fixed cheeks and glabella is one of the Olenidæ.

[2] Nat. Classification of the Trilobites, p. 183.

[3] Oldest in that sense that they are the descendants of an archaic precambrian stock, the chief characteristics of which they have retained in the main unchanged and persisting long ages after the close of the Cambrian times, some, as Agnostus, continuing high up in the Lower Silurian.

II. Blind trilobites with facial ridge.

This large division embraces the second and third groups or, with a few exceptions, all the rest of the Cambrian trilobites on account of a feature in the cephalic sculpture common to them all, though widely different as to its first origin in both. What forms the prominent and common characteristic of these two groups is the presence of the facial ridge, which emanates from the basis or the front of the foremost segment of the glabella and in a great variety of different shapes continues backwards near to the posterior border of the head. It has received several names as eye-line, palpebral lobe, ocular ridge, eye-lobe, ocular fillet (Matthew). In German it is named Augen-leiste, in French filet (Barrande) and in Swedish ögonlist.[4] Some authors make a difference between the more narrow part, calling it eye line, and the thicker posterior node, which they name the palpebral lobe proper.

[4] That name is the most current amongst the swedish authors, together with »palpebrallob»; Holm says ögonlob and frontallob.

As this peculiar ridge exists before any facial suture has made its appearance and separated the head shield in five parts, viz. the median glabellar part, the two fixed cheeks and the two free cheeks, and as it occurs in genera which never possessed any facial suture, and where no eye ever was formed, it is not adequate to call it an ocular ridge etc. the more so, as it, at least during a long series of genera succeeding each other, has had no connection whatever with any eye. I therefore propose to call it facial ridge (in swedish faciallist). It occurs on the head of almost all Cambrian trilobites, excepting the archaic ones, and it is retained in the later Cambrian Peltura, Sphærophthalmus etc., which have real, compound eyes, as well as in a few Lower Silurian genera as Triarthrus, Pliomera,[5] Euloma, in the Upper Silurian Arethusina and Acidaspis and most persisting in Harpes, ranging from the Lower Silurian Lower Red Orthoceratite Limestone into the middle Devonian beds.

[5] Pl. Törnquisti Holm.

It presents itself in the most variable shapes, and as it in fact can be followed through its development in the oldest known species, it is suitable to begin its description together with the characteristics peculiar to the oldest or second group in this large subdivision, that of the Olenellidæ.

Group 1. The Olenellidæ.

In this group we have two families of different age, the older, less developed the Olenellidæ proper, and the younger the Paradoxidæ. The former consists of the genera Olenellus, Holmia, Mesonacis and Schmidtia, and we shall attend to them first. They have no facial suture[6] and consequently a tripartite head shield like the Archaic ones, no eyes, but there is that strongly developed and most characteristic facial ridge. As seen in Olenellus Thompsoni[7] the crescent like ridge starts as a direct outflow from the base of the first segment of the glabella, and is in direct continuation with it as an integral part. It is roundbacked, regularly curved and at its starting point as broad as one of the posterior glabellar segments. It is regularly semilunate, tapers posteriorly and ends near the occipital segment. From the narrow second segment of the glabella, inside the just described larger ridge, a smaller ridge emanates, broader and flatter than the former, slightly curved, and ends between the third and fourth glabellar segment. Already in Holmia Kjerulfi the ridge is modified. There it is only one ridge, the anterior one, nearly as doubled through a shallow groove running along its back. The second one has dwindled away so as to be seen only as a narrow stripe near the occipital segment and ending outside this in a point. The anterior ridge, represented by Holm as consisting of two nearly independent parts, is indeed in one piece, though its dorsal groove sometimes is deep, and it is with its total breadth joint to the first glabellar segment. Along its outer edge, where it lies close to the cheek, a narrow slit runs, and I suppose that it is to be considered as the first indication of the forming of a facial suture, which however does not reach longer than the ridge. As in Paradoxides it is so tight to the cheek, that there has been no place for an eye. It is no accidental break, its edges being too regular and unbroken in all specimens. This ridge is by far much shorter than in Olenellus and terminates opposite the third segment.

[6] Beecher Natural Classification of the Trilobites, p. 191, pretends that in Olenellus and Holmia real sutures »in a condition of symphysis» occur. He seems to deny the facial sutures and to accept as »real sutures» the »internal sutures» described by Holm in Olenellus Kjerulfi. It is highly doubtful if these interior elevated lines are to be regarded as sutures. They are indeed no sutures, but in reality elevated linear ridges, inclosing, as it were, narrow canals. The real sutures known, the facial sutures, never form elevated lines, be it on the outside of the head or on the inside. Probably these lines are derived from some now unknown interior organization and it may be fit to remind of the somewhat similar though more numerous linear canals of the branchiæ on the interior surface of the great head shield of Apus. (Zaddach De Apodis cancriformis Anatome ... pl. II fig. 1) or what Huxley (Anatomy Invertebr. Animals p. 281) calls the convoluted »shell gland» in the carapace. A quite different structure is what I suppose to be the incipient facial suture in Holmia.

[7] Walcott in 10th Ann. Rept. U. S. Geol. Survey, pl. 82 f. 2.

Larva of Olenellus asaphoides, chiefly according to
Ford and Walcott.

1. Hypothetical figure, based on fig. 2. represents a stage preceding the next. There are three pair of pleura and three segments of the rhachis.	2. Copy of Walcott's fig. 5 in »Bullet. 1886», pl. XVII. Three pleura at right, two at left, the larger formed through fusion of the two hindmost in fig. 1.	3. Copy of Walcott's fig. 6: the second and third pleura have been united into one.	4. Hypothetical figure as a further development, following upon the stage represented in fig. 3. Fusion completed between all posterior plectra (2, 3, 4), thus forming a single large pleuron composed of the three mentioned.
5. Copy of Ford's fig. 1, pl. IV, Amer. Journ. Science 1877, enlarged to the same size as Walcott's. First sign of the pygidium.	6. Copy of Ford's fig. 2 (1877) enlarged. The pygidium has been added to the head shield.		7. Copy of Ford's fig. 3, enlarged.
8. Copy of Ford's fig. 1 page 251 in Am. Journ. Oct. 1881, enlarged.	9. Copy of Ford's fig. 2 page 251. Amer. Journ. Science 1881. Slightly enlarged.		10. Copy of Ford's figure 5, pl. IV, Amer. Journ. Sci. 1877, slightly enlarged.

Already Linnarsson observed the second ridge as he tells in a paper where he describes Olen. Kjerulfi for the first time.[8] It has been called an »ornamental» spine, but in the following we shall learn what it really is. This ridge in connection with its spine has not been observed in any other of the Olenellidæ, at least not in their adult stage.

[8] Öfversigt Vet. Ak. Förhandl. 1871, tafl. XVI fig. 1.

Thanks to the discovery of very early larval stages of the American Olenellus asaphoides, which Ford[9] and Walcott[10] have described and figured, we can through combination of these decipher the development and signification of the facial ridges. To facilitate my explanation I here join a series of cuts from the earliest stage to the more developed, with addition of two schematic stages to complete in a certain degree the series of Walcott and Ford.[11] See figures [p. 13].

[9] In American Journ. of Science 1877 p. 265 and 1881 p. 250.

[10] Bullet. U. S. Geol. Survey N:o 30, 1886, pl. 17 fig. 5-6.—Tenth Annual Rept. U. S. Geol. Survey, 1888-89, printed 1890, plate 88, fig. 1, 1 a.

[11] In his paper on »The larval stages of Trilobites» p. 175 Beecher gives a new figure (f. 6), original from Ford's collection, of the larva of Ol. asaphoides, but it is so sketchy that I cannot with certainty make out what it means. I cannot agree with him when he speaks of free cheeks and eyes in these and he is completely wrong when he says that the outer pair of spines belong to the free cheeks etc. (p. 176).

The [figure 1] has been hypothetically composed as a deduction from [fig. 2], which presupposes an earlier stage of development like that in [fig. 1], when there existed three or four pair of lateral appendages in the larva. This then consists of a central portion of five segments. The large anterior crescentlike segment does not, however, show any distinction between a central part and lateral appendages, it is nearly as large as the three next taken together and its backwards bent side horns embrace the next two posterior segments and attain with their narrow pointed tips the back of the fourth segment. The central portion consists of five segments, when the somewhat not well definite posterior marginal segment is taken in account. Each of these segments excepting the fifth one has lateral appendages, those of the second and third segment being quite as broad as the central part and bent backwards in a curve ending in a small pointed spine. The lateral appendages of the fourth segment are largest of all, more than double the length of the two next in front, triangular and standing out beyond the posterior border of the shell as a broad spine.

In [fig. 2] a great change has set in. There is no distinction between the lateral appendages of the third and fourth segments at left. These two have been fused together, they have united, so as to make the left triangular spine look larger than it was originally. The appendages of the right side are still in the same state as before. But this fusion of the lateral appendages also takes place in another direction, as shown in another specimen ([fig. 3]), so that the second and third appendages on both sides coalesce into one piece. Now it is easy to imagine that at last a complete fusion has set in between all lateral appendages and that instead of the original three on each side, there is only one large piece, reaching beyond the shell as a broad spine, as represented in the hypothetical [figure 4].

In the [figure 3] a progressive change is also seen in the transformation of the first central segment. From occupying the whole foremost space of the shell it has been lessened in size, more distantiated from the anterior border of the shell and rounded off, more prominent and definite from the lateral appendages, which have become narrow, though of the same length as before. The two first central segments seem to have been united into one.

Between the hypothetical [figure 4] and [fig. 5], there is evidently a great lacuna, not yet filled up. In the interval of time the two appendages, which we saw in [fig. 4], have been much modified, the posterior one having lost so much in bulk, and the anterior being lengthened and stretching out beyond the border of the shell in a narrow spine alongside the posterior one. The central segmented part has now assumed a shape, which on comparing it with the following stages of development makes its true nature evident and that it indeed is what in the adult animal becomes the glabella. The meaning of the previous stages then also is easily understood. The central segmental piece in them is the glabella or we may, as Bernard already has done, call it for rhachis and the side appendages for pleura, as this little larva represents the whole body of the future trilobite, and embodies all its parts in nuce. Through the great changes which these pleura undergo, it results, as we have seen, that two pair vanish, being incorporated with the large fourth pair and that only two rest for a while, the anterior one being the so much renowned frontal-lob or eye lobe and the second one the so called »ornamental spine», which in fact is a compound of the original second third and fourth pleura of the corresponding segments.

It is to be borne in mind that this larva, which represents the future trilobite in its earliest stage, is nothing but the head, or what in the adult takes the place of the head, and especially its dorsal surface and that it thus solely consists of the future head.

In the [figure 5] (Ford's fig. 1.) there are the first signs of the pygidium coming and in the [fig. 6] it is well developed, but the thorax is still non apparent. In the stage [fig. 6] both pleura have increased in length and the compound one also in bulk. They project with spiny points beside the beginning pygidium and the anterior pleura have united across the first segment of the glabella through a narrow ridge, which seems to cut that segment in two. The sequence in the order of development or growth thus is first the head, then the pygidium and last the thorax. At least it is so in these the oldest of all trilobites. But in nearly all trilobites of which there are good data, the head is the part first developed.

Evidently a large hiatus exists between the stages represented in figs. [6] and [7] (Ford's figs. 2 & 3, 1877), in the latter of which the animal, though not adult, has had the thorax and pygidium added to the head. The modifications in the size of the pleura are the chief changes. The anterior pair is reduced and retired within the posterior border of the head forming a semilunar arch joined with the occipital ridge in an angular bend. The posterior pair is enlarged and its spine is by and by reduced (figs. [8], [9], [10]) till it quite disappears and only the wide semicircular field between the first pleuron and the glabella is left behind as a remnant of its dilated body. It is to be remarked, that while in the plurality of the adult Olenellidæ all traces of the spiny projections of the second pleuron have vanished, they are still retained in the adult Holmia Kjerulfi, though not in the American Holmiæ, and thus give it at the same time a more ancient and a more larval stamp. The shallow groove along the back of its first pleuron indicates strongly its pleural nature, as the thoracic pleura commonly are divided through such grooves. The same peculiarity is also observable in several of the American Olenellidæ. It is much the same with the posterior pleuron, the pleural nature of which is revealed through its spine, that is homologous and identical with the spiny terminations of the thoracic pleura. We have thus through the remarkable finds of Ford and Walcott combined received an explanation of the morphological origin and nature of the facial ridge, the so called eye lobe and found that it has nothing whatever of the character of a visual organ. But it must be borne in mind, that these developmental changes are peculiar only to the Olenellidæ, the origin of the facial ridge in the later trilobites is, as we shall see, quite a different one.

The Olenellidæ belong chiefly to the oldest of the Cambrian beds with trilobites, and none of them has as yet been found higher up.

Next in order of evolution we have the important tribe of the Paradoxidæ. These are preeminently distinguished from the Olenellidæ through the well developed facial suture, which without exception in them all runs outside the facial ridge and separates this from the free cheek.[12] This is a great step forwards in the evolution and establishes the fact, demonstrable also by other evidence, that the formation of the facial suture is subsequent to the appearance of the facial ridge. This preexisting ridge seems to have had no small influence on the development of the suture, it checked its progress from the front or from the sides toward the fixed cheeks and directed its course against the genal angles. It lay as a protection for the glabella against this disseverance, causing the separating line to run along its outside.

[12] They have thus a quinquepartite cephalic shield, as the later trilobites.

This group consists of the genus Paradoxides proper, as well as of Centropleura, Metadoxides and Hydrocephalus, if this is an adult form and not the larva of an unknown Paradoxides. Perhaps such forms as »Conocephalites» Emmrichi Barrande, as well as Anomocare limbatum, An. excavatum, Bathyuriscus and Dolichometopus may on account of the shape of their facial ridges be considered as related to the Paradoxidæ. But this must be left for coming researches to decide. Some American Cambrian forms also share in this characteristic and may upon closer inspection be ranged here. So Zacanthoides. In these as in the true Paradoxidæ the facial suture follows the ridge along its whole length, while in the trilobites of the third group the facial suture is in contact only with the posterior end of the ridge, the so called eye lobe. Remopleurides does not show characters, that as Beecher thinks, could unite it with the Paradoxidæ. These are blind and Remopleurides has well developed eyes and an organization that gives it an isolated position in the system.

The facial ridge continues in a great variety of shapes, short or long, but always forming the fraction of a circle, always of nearly equal thickness, only slightly tapering towards one of the extremities, and always when in direct connection with the glabella, starting from the base of its foremost, largest segment. As a rule the ridge is more developed in the young or larval individuals, continuing from the glabella to near the posterior cephalic border in an uninterrupted arch[13] quite as in several of the adult Olenellidæ of the oldest Cambrian. It can be taken as granted that its origin is the same as in the Olenellidæ though at present the only evidence at hand is the small larva of Paradoxides oelandicus, which Linnarsson called Parad. aculeatus.[14] In this we see the anterior pleuron or the facial ridge alone present, elongated downwards like the same pleuron in the figure 6 of Olenellus asaphoides and terminating like this in a fine spine stretching backwards outside the posterior border. Of the second pair of pleura there is nothing to be seen. This must then have been aborted at an earlier stage than in the Olenellidæ.

[13] G. F. Matthew had before me, as I now find, pointed out this distinction in his memoir »Illustrations of the Fauna of St. John» N:o IV, p. 163. When he speaks of »the embryonic stage» in this and other passages he evidently means »larval stages», as the embryonic stages of necessity must remain unknown to us. (Later remark.)

[14] Om faunan i lagren med Paradoxides ölandicus (1877), p. 359, pl. 14, f. 11.

The connexion between the free cheeks and the middle part of the head has been very lax not only in the Paradoxidæ, but on the whole in nearly all Cambrian trilobites with free cheeks. When the free cheek is dissevered it shows no trace of the ridge, there is only a large scallop on the spot where it embraced the ridge. In the Paradoxidæ the rim of the indenture and the ridge are in so close contact that there is not the least place for an eye between them, as can be seen in the few specimens with a complete head. In all oculate trilobites again without any exception the facial suture separates that part of the eye which is the real visual organ with corneal facets, from the interior often elevated portion, opposite it, the so called palpebral lobe. The eye is always placed on the free cheek,[15] the lobe again always on the fixed cheek of the head shield. No real eye exists without the palpebral lobe, and, on the other hand, that part of the facial ridge which later in the development changes to a palpebral lobe, occurs alone without any eye in a great number of Cambrian trilobites, and consequently these are blind and such is the case with the Paradoxidæ and a great number of the succeeding.

[15] Excepting in Harpes, which has no free cheeks.

There is not the least evidence to support the suggestion that the »ocular ridge» is homologous, with the eye of Apus[16] and that the real crystal cones lay sunk beneath the surface in a »water sac». As we, for instance, in Peltura have an »ocular ridge» (= facial ridge mihi) on the fixed cheek and opposite its posterior extremity, the »eye lobe», a real eye with facets on the free cheek it is not likely to suppose that the »ocular ridge» nor the »eye lobe» ever functioned as a visual organ or that two widely different sorts of eyes were placed in closest vicinity opposite each other.

[16] Bernard, The systematic position of the Trilobites. Qu. Journ. Geol. Soc. 1894, p. 411.

It may be worth while here to remind of the great similarities, whether analogous or homologous, in the formation of the superior surface of the head in the trilobites and the embryons and the newly hatched larva of Limulus. The latter have the head shield separated into five portions, partly through a facial suture which, as in Peltura, Dalmanites and others, forms a continuous line around the glabella. This suture divides the eye node in a similar way as in the trilobites, that is, sectioning it in two parts, of which one adheres to the central fixed cheeks and the other to the free cheeks.[17] And to judge by the figures of Kingsley the former, the whitish moiety is the first developed and sometimes for a while quite alone as the facial ridge of the trilobites and probably also anterior to the suture, as this is not complete at this stage. This white node reminds of the small facial ridge in Arionellus ceticephalus Barr.

[17] Packard, Development of Limulus polyphemus. Memoirs Boston Soc. Nat. H. vol. II, pl. V, fig. 25. Nothing is said about the exterior structure of the eyes.

Dohrn, Zur Embryologie and Morphologie des Limulus Polyphemus. Jenaische Zeitschrift 1871. A very good figure (pl. XIV, f. 4) shows clearly the two parts of the eyes, the interior one being larger.

Kingsley also (Devel. of Limulus, Journal of Morphology, vol. 7, 1892, pl. VI, fig. 34) has in the last larval stage the suture and the eye in two parts, of which one is white lying inside the suture and the eye proper, black, outside.

As Kishinouya[18] has already pointed out and as I have anticipated above the head of the most developed trilobites in their adult state, and the head of the larval Limulus consists of five parts, viz. 1) the glabella in the centre, 2 & 3) the fixed cheeks, 4 & 5) the free cheeks. An elevated ridge in the adult Limulus shows where the suture once lay and it is on the outside thereof that the eye of the adult is placed. What other authors call the ridge or the eye ridge in Limulus, Kishinouya rightly names a suture.

[18] Journal College of Science Tokio, vol. V, p. 53, 1892.

It was the renowned Swedish naturalist Wahlenberg, who first recognized the importance of the facial suture, which he called »linea ocularis»,[19] but to another Swedish palæontologist Dalman[20] the exact definition of this suture is due, to which he gave the name still in use. He expressly remarks that the suture crosses the ocular node and limits the outside of the »lobus palpebralis» and he makes a clear distinction between that lobe and the »tuberculi and eminentiæ oculares» (= facial ridge) of which he says that they are the more or less evident elevations situated in the blind Palæades on the place of the eyes (in which he is wrong) »and which perhaps are an indication of such organs». But then he says doubtfully (p. 255) »tuberculorum ocularium veram naturam determinare haud ausi sumus, etsi oculorum formam sat bene exhibere videantur» and he adds concerning Paradoxides »oculi nulli, eorum loco autem tuberculi duo».

[19] Additamenta quædam ad petrificata telluris Suecana, in Acta Upsaliensia, vol VIII, 1821, p. 294.

[20] Vet.-Akad. Handl. 1826 p. 126.

Group 2. The Olenidæ and related families.

Next in the ascending order we have the largest group of trilobites in the Cambrian, of which the greatest part is formed by the Olenidæ. A facial ridge different in shape and different in development from that in the former group characterizes them. Unlike the semilunate ridge of the Olenellidæ it issues mostly from the front of the first segment of the glabella and goes generally backwards till it meets the facial suture. It is narrow and fine as a thread, but for the rest assumes a great variety of forms. It may be curved as a circle segment as in Sao, Liostracus, it may be long and straight, standing out in a right angle to the glabella as in Eurycare, it may be short and straight in an acute angle to the glabella as in Parabolina and so forth. In the same genus, as for instance Olenus, straight and curved ridges occur in the various species, and consequently the form of this sort of ridge cannot be used as a generic character. It moreover differs from the ridge in the Olenellidæ and the Paradoxidæ in having the posterior extremity widened as a tubercle or node, which commonly has been called the »palpebral lobe», while in the older groups the ridge is only at the most a little pointed in both ends or of equal thickness.

Some persistent archaic genera as Conocoryphe and Elyx inform us how this ridge has originated and how widely different it is from that in the Olenellidæ. On the surface of their head shield no ridge is visible, but on the interior side of it we behold, as the figures show ([Pl. VI, figs. 43 & 44]) on each side of the glabella a ramifying system as of the most minute vessels, which spread and cover the whole surface of the cheeks. This reticulation issues from a main trunk that goes nearly straight in an oblique direction from the foremost segment of the glabella and emits narrow branches forwards and backwards on its both sides. And these branches go on dividing till they occupy the entire interior surface of the fixed cheeks. The figures representing this are taken from Elyx laticeps [fig. 43], and an almost similar image has been obtained from a species of Conocoryphe [fig. 44].

Now the question may be asked, what does this network of branching canals mean? I think we cannot gain a more plausible answer than that given us through the inspection of the soft parts that lie hidden behind the glabella of Limulus and its fixed cheeks, the only living crustacean which offers the greatest homologies with the head shield of the trilobites. Next below the shell of the glabella and the cheeks of Limulus there is a complicated stratum of muscles and behind this the heart and the great central circulatory system spreading from that centrum towards the sides of the head shield, the vessels being the more fine and minute, the more they are elongated from the centre. I now suppose that likewise in these trilobites the narrow and prominent glabella has been the receptacle not only of a strong mass of muscles, but also for the central part of the circulation. This centre has there sent out two relatively strong ducts or canals, one on each side of the glabella feeding all soft tissues inside and near the cheeks and probably also other important parts of the body. It is also clearly seen in Elyx how the orifice of the main trunc opens in the hollow of the glabellar apex.

In all crustaceans, as far as is now known, their more or less hard calcareous or chitinous skeleton is moulded by the subjacent tissues and glands. The sculpture of the surface consequently is an outcome of the fashioning procedures showing what has been going on below and what is still going on. If we then on the exterior surface of the head shield of the trilobites see the radiating lines in relief we must conclude that they are due to a subjacent system of almost capillary vessels causing hollow impressions on the inside and elevated ridges exteriorly. In Elyx the vessels have made no strong impression as to be visible on the outside. In a couple of American Conocoryphæ again (C. reticulata and C. trilineata) the main trunc of the vessels has formed a short faintly elevated ridge. In Solenopleura ([Pl. VI, f. 45]) the facial ridge has been fully developed, and by the casts of the inside it is clearly seen that the main trunc of the vessels makes the inside of the ridge and has been much incrassated. On the other hand the smaller branches issuing from it have been reduced in size and number, but are still visible. Thus, if I am right, the two, the facial ridge and the main trunc, are in causal connection and the former has been moulded by the later, when it had gained in size sufficiently, and when a richer affluxion of nutritive fluid was directed backwards towards the point where the eye at last originated. The further changes in this ridge thus are related to the development of the eye. As far as I am aware, there are as yet no data to tell us whether the appendages or pleura of the glabella in the larval (or primordial) Olenellidæ have the same origin as the ridge of the Olenidæ or are homologous with it. Perhaps it may be so in respect to the foremost one, which also is persisting. It is, however, not known how the pleura in the olenellid glabella have been developed, whereas it is well ascertained by the growth of the glabella in Sao, Liostracus and others that such pleura never have been developed in them.

What the phylogenetic evolution has taught us concerning the formation of the facial ridge is confirmed by the ontogenetic development of individuals of some species as for instance Sao hirsuta Barr., Liostracus sp. Brög. and others.

This development can be followed in detail through the excellent figures of the larval stages of Sao hirsuta which Barrande has given in his magnificent work, vol. I, pl. 7. In the first stage figured (figs. 1 d-e) the whole animal consists only of the head shield which is completely smooth, the glabella scarcely segmented, no facial ridge, no facial suture. In the second stage, according to me (figs. 2-4 a), the glabella has become distinctly segmented, and the pygidium and partly the thorax have been added. In the third stage, 4 c, d-9, one small narrow string exits from each side of the front of the glabella, making the first faint beginning of the facial ridge. They form a right angle with the glabella. In continued growth they become by degrees a little more curved (figs. 5 c, 6 b) and the lengthened ridge bends parallel to the outer lateral margin of the head (fig. 9 b). In fig. 9 d it has become so far complete that it reaches nearly back to the posterior cephalic margin, but is still of the same narrowness all along. In the fourth stage, in a specimen (fig. 10 a) of 3 mm. in length, the facial suture makes its first appearance, setting in from the posterior margin of the head and meeting in its forward progress the posterior extremity of the facial ridge which now begins to swell out. It seems that both have a mutual influence on each other, the suture being deviated from a straight course[21] to take a bend outside the ridge, and the extremity of the ridge again at this contact to increase in size so as to form the elongated tubercle, often called palpebral lobe (fig. 14 b). This now augments in the same rate as the whole body. The characters of the four stages of the development of the larva consequently are: 1:o The archaic stage, only head shield with ridgelike glabella. 2:o The coming and growth of the pygidium and the complete segmentation of the glabella. 3:o The coming and growth of the facial ridge. 4:o The coming and development of the facial suture.

[21] In Trinucleus and Ampyx where there is no facial ridge, the suture has a straight direction along the lateral margins.

Barrande regards the whole ridge as a prolongation of the eye and the tubercle at its posterior extremity as the eye itself.[22] But, again, in page 399 he says »Sa surface (of the eye) est toujours mal conservée, pour nous permettre de voir si elle était réticulée.» And he also confounds the ridge, »filet» as he names it, with the eye itself, and the first faint beginnings of this ridge in his fourth stage he considers as the eye.[23] By a partly schematical figure of the free cheek (fig. 25) he places the eye on this cheek, and in the same manner in fig. 29, »restaurée d'après divers fragments» he figures a reticulated surface of the eye on the free cheek, outside the tubercle. I have sought for a reticulated surface on sufficiently good specimens, but never found any, and I must consider Sao as one of the blind genera. Barrande himself also in the table on the eyes of the trilobites places Sao in the group with »Surface visuelle inconnue», p. 131.

[22] Page 383 »l'œil argue est prolongé par un filet en relief, vers le front de la glabelle».

[23] p. 389.

(0 is new, added here to supplement fig. I.)

Brögger has also succeeded[24] in finding a series of small larva, which he considers as belonging to a species of Liostracus. As the figures drawn by Brögger twenty five years ago may now be very little known, I here reproduce them with the kind permission of the Editors of »Geologiska Föreningens Förhandlingar», where they were published in 1875. This development proceeds nearly upon the same plan as in Sao. The first stage, however, (I) seems to be much earlier than any of Sao, the rhachis or glabella consisting of an unsegmented ridge of more primitive appearance. Before this first stage of Brögger's larva a still older phase of development can be imagined, (0) a simple rounded, smooth head shield without any indication of a glabella at all. This stage might correspond to the head of certain species of Agnostus, as A. glandiformis, A. nudus, which have no glabella. This stage 0 is also valid for the larva of Sao. Several stages are evidently wanting between I and II, in the later of which the thickened glabella is divided in four segments. In III we have six segments, all these three stages consisting only of the ovate head with narrow fixed cheeks. In IV the pygidium has been added to the primitive head, but the segments of the glabella have been reduced to four and in V slightly altered in shape In VI, again, we see the head with five glabellar segments and scarcely the first sign of the facial ridge. Between VI and VII there must be links missing, as the change can not be so abrupt, and likewise between VIII and IX as in VIII there are still no free cheeks nor any facial ridge. This interesting discovery of Brögger confirms, together with those of several other authors[25], the supposition that the development of the later Cambrian and older Silurian forms is a quite different one from that of the Olenellidæ and the Paradoxidæ. They have a rhachis, but no pleura proper, as the single facial ridge has a quite different signification and appears at a comparatively much later stage than the facial ridge of the Olenellidæ, which is present from the earliest stages known.[26]

[24] Fossiler fra Öxna og Klettna, Geol. För. Förhandl. 1875, p. 572, pl. 25, fig. I-X.

[25] Foremost among these stands Matthew in »Illustrations of the Fauna of St. John» IV, where he, p. 143, pl. II figs. 1 f. etc., describes a few stages very like those given by Brögger, so the glabella as an unsegmented, narrow ridge etc.

[26] A deviating form of the ridges is shown by »Liostracus» tener Hartt, Acad. Geol. 2d Ed. p. 652 (see also Matthew Illustrations of the Fauna of St. John [1887] p. 132 p. 1 f. 3 a-3 c). Beside the usual facial ridge there is a second pair of ridges between the first and the glabella, arching in an opposite direction. See also Walcott, Bulletin U. S. Geol. Survey, N:o 10, 1884 pl. V, figs 6, 6 a, 6 b, new figures and copy of Hartt's description.

This form of facial ridge, although prevalent in the middle and later Cambrian times, dates back so far that genera coeval with Holmia, viz. Ellipsocephalus and Arionellus show it along the facial suture. This early occurrence of the ridge coeval with the less developed Olenellidæ leads to the assumption of a different origin of these trilobites as a branch, which already far back in the oldest Cambrian or precambrian times had deviated from a common ancestor and I have tried to give a view in tables of these two coordinated lines of evolution further on, at pages 24 and 25. The most remarkable genera which, as far as I have found, belong to this fourth group are Ellipsocephalus, Arionellus, Liostracus, Olenus, Leptoplastus, Parabolina, Corynexochus, Parabolinella, Sao, Ptychoparia, Doropyge, Oryctocephalus etc.

I have not been able with an absolute degree of certainty to recognize whether some of these genera now enumerated, have been oculate or blind like Sao, like the Paradoxidæ and similar. The precarious state of preservation prevents all definite conclusions in that respect. It seems, however, that the evidence gathered through the examination of numerous specimens rather points in a negative direction. As the free cheeks in these old Cambrian trilobites have been in a very loose connection with the fixed cheeks and generally deciduous, contrary to the condition in the Silurian ones, it is in many instances very difficult to tell whether species with facial ridge, especially those from the earlier Olenus schists have been blind or provided with eyes.

The order of succession of the genera in the Swedish uppermost Cambrian, in the Olenid slates is, according to S. A. Tullberg's researches[27] at Andrarum in Scania as follows:

1. Parabolina (oldest division of the Olenus slates).
Olenus.
Liostracus.
2. Eurycare.
Leptoplastus.
3. Peltura.
Sphærophthalmus.
Ctenopyge.
4. Cyclognathus (uppermost).
Acerocare.

[27] Om Agnostusarterna vid Andrarum.

It is already in the second division that we find the earliest oculate genus in Eurycare and in the higher strata. Sphærophthalmus and Ctenopyge with enormous hemispherical eye balls, while Olenus and Parabolina, their earliest predecessors, probably were blind. We have not amongst hundreds of specimens of these genera found a single specimen showing an ocular globe covered with facets. Olenus and the nearly related genus Parabolina are found in innumerable specimens in the thinly laminated alumschists of Scania and other provinces of Sweden. But rarely a perfect head shield, or nearly so, is to be found with the free cheeks in place. If so, the semicircular scallop in the free cheek is entirely filled up by the posterior lobe of the facial ridge and there is no place left for any eye.—If we, again, find a non compressed glabella and fixed cheeks likewise, the facial lobe (-»eye-lobe») is in some elevated so much as to leave a little space between it and the scallop of the free cheek, which space must have been an empty lacuna if not filled up by an eye ball. But it may also be that the free cheek has been somewhat put out of its order and that consequently some space has been left between it and the free cheek. It is quite as much with older genera, as Solenopleura especially, in which the posterior lobe of the facial ridge (vulgo »eye lobe») has attained a great development, and in which one just could expect to find a sphærical eye resting between the elevated lobe and the scallop. The elevated rim of this scallop does not in fact constitute a proof for its having clasped an eye as the elevated scallop in Sphærophthalmus did. In Paradoxides, again, in Ellipsocephalus, where there is absolutely no trace of an eye ever having been present, whenever you succeed to find the free cheek in juxtaposition with the fixed cheek it is evident that the elevation of the scallop rim is due to the impact of the posterior lobe of the facial ridge. In many specimens of Dolichometopus and Corynexochus etc. no free cheeks have ever been found and to judge by the shape of the facial ridge it may be concluded that these also were deprived of eyes.

On seeing this great number of trilobites, that on account of their organization must be considered as blind, the first suggestion that strikes the mind, is that they must have lived in abyssal depths of the Cambrian sea, where the most intensive darkness prevailed. Nor does the nature of the strata contradict such an assumption as evidently this fine sediment must have been deposited far beyond the reach of the influence of the wave motion, a depth amounting to more than a thousand metres as now calculated, for else it could not have preserved unbroken such delicate parts of organisms as that free cheek of Ctenopyge pecten with its extraordinary long and delicate horn, figured on plate III fig. 27 and many others. But it seems incredible that such a state of things should have prevailed during the deposition of all the Cambrian strata, although they in Sweden amount to only 160 feet in thickness, according to the evaluation of S. A. Tullberg, not considering what has been lost through denudation. The length and duration in time can in this instance not be measured by the thickness of the beds, but by the great changes in the faunas which there have succeeded one another. The physical conditions, to judge by the composition of the rocks, seem in the main to have lasted during immeasurable periods and still the fauna has changed in no little degree. During that enormous length of time, embracing in Sweden eight well separated periods, there must, however, have been minor changes in the conditions of depth and consequently in the nature of the depositions. To a certain extent the physical agents must have influenced the organization of the animals, but not essentially. There are sure evidences of another factor being the chief agent and that is the evolution.

We have from the lowest Olenellus beds to the lowest Lower Silurian strata followed the clear traces of the changes from the eyeless Olenellidæ past the Paradoxidæ with the facial suture to the great tribe of the Olenidæ in which the facial ridge with its lobe is so prominent, and to the oculate genera at the top of the formation.

As stated above there seems to be evidence enough for accepting two different lines of evolution in these the oldest trilobites. For the oldest, Olenellus is the type and for the second Sao may be taken as the representative.

These two evolutional lines could be represented with their phases in chronological succession as follows:

A. The Olenellus line.

1. The trilobite consisting only of the head with rhachis and pleuræ, no sutures, no ridge, no eyes. Adult forms precambrian. Corresponding larval forms (Olenellus asaphoides) in the Lowest Cambrian. Still older and simpler forms may be presupposed as preceding these.

2. Semilunate facial ridge on the tripartite head-shield, no sutures, no eyes, but a hypostoma fused to the rostrum of the head and provided with maculæ. The adult animal with thorax and pygidium in the oldest Cambrium, the Olenellus zone. Genera: Olenellus, Holmia (with beginning suture), Mesonacis, Schmidtia.

3. The head quinquepartite with facial suture, short semilunate ridge in the adult, a long ridge in the young, no eyes. The genera occur in the Paradoxides-zones of the Cambrian formation. These genera are Paradoxides, Centropleura, Metadoxides, Hydrocephalus, which are direct descendants of the Olenellidæ.

This line of evolution, in which the species never acquired eyes, was extinct in the sixth zone of the Swedish Cambrian, Centropleura Lovéni being the last.

B. The Sao line.

I call this so because we see in the development of Sao, as represented by Barrande, most of the different' phases in evolution, which the ridge bearing trilobites of this second group have experienced during the Cambrian times.

I.

The body consists only of the head, neither pygidium, nor thorax yet developed.

1. Psilocephali (ψιλος, bald), primeval, precambrian, adult stage not found, supposed to be like the head shield of Agnostus glandiformis and A. biplicatus quite round and bare without any glabella nor sutures. The corresponding stage in development also wanted.

2. Glabellate. The entire animal only head with a mesial ridge, which at first is entire or unsegmented and later is metamorphosed into the segmented glabella.

a. unsegmented., Precambrian, adult form unknown, supposed to be like the first larval stage of Liostracus according to the figure I of Brögger and also as the head of Agnostus parvifrons Linnarsson. Corresponding larval stages Liostracus (I).

b. segmented. Probably precambrian, not represented as adult in the Cambrian strata. Supposed to be like the larval forms II and III of Brögger's Liostracus, consisting of the round or oval head shield and the segmented glabella. Also Barrande's first stage of Sao (Barr., fig. 1 pl. 7).

As a transition to the next phase forms may be imagined having head and the beginning pygidium, nearly as Agnostus atavus, minus the thorax. The corresponding larval stages are Liostracus (Brögger's figures IV, V, VI) and Sao (Barrande, pl. 7 fig. 2-4 a, b). Nearly so, though the pygidium is more developed, are the larva of Agn. bibullatus and Agn. nudus (Barr., pl. 49) both without thorax.

II.

These have the three chief parts of the body developed.

3. The facial ridge. Cambrian, adult with glabella and facial ridges, short or long, emanating from the top of the glabella, thorax and pygidium. Several American Conocoryphæ. Corresponding larval stage, Sao, stage 3, Barr. pl. 7 figs 4 d-9.

4. Suture. A suture dividing the fixed cheeks in two pair viz. two fixed cheeks and two free cheeks. Fully developed facial ridge. Oldest known adult forms are Ellipsocephalus, Arionellus, already in the Lowest Cambrian, the Olenellus beds, what presupposes a long, antecedent lineage far back in the precambrian times. Corresponding larval stage in Sao, Barr. pl. 7 figs 10-13. The plurality of Cambrian trilobites belong here. An intermediate stage leading to the next is seen in such forms in which the ridge posteriorly is widened into the »eye lobe», which rests in the scallop of the free cheek. So it is in Liostracus and many others besides. Solenopleura possibly oculate.