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.