36 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



les 011 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 Cru- 

 stacea, at least those of the head, elevated small platforms, so in the Trilobites, of which I 

 have excellent specimens in a Bumastus and others. ()n the inside of the head of Limulus 

 they are faintly elevated patches. The small hollows on the inside of the hypostoma formed 

 b v the maculae are indeed the soekcts in which the soft parts of these more or less de- 

 veloped hypostomic eyes were sheltered. But there is still a featnre in the hypostoma of 

 Acidaspis which merits our attention and which perhaps may have a significance akin to 

 that of the maculae. 



The hypostoma is square, with two short pointed wings, onc on each side of the 

 slightlv 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 s;ime direction, distally. 

 though a little more inside and uiiconnected with them there are two small grooves, the 

 bottom of which consists of a shell substance of different colour and strueture than the 

 other parts. Having been a little ground and seen in transmitted light it exhibits the 

 shape of a elub and a homogenous yellow spöt, tapering posteriorly and swelling ont 

 distally (Pl. I fig. 4). It must be left an open question whether these maculce share in 

 the nature of visual organs as the quite different maculae 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 maculae, 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 qua- 

 dratic or rectangular shape, but the two short grooves, which may be expected to contain 

 the maculas, are placed midways between the anterior and the posterior margins, 

 nearly as the macuhe bearing grooves of other trilobites. We have however not had 

 material for pursuing our researches in this genus, the other species of the Swedish Acid- 

 aspidaj being unknown as to their hypostoma. 



It may here be added an observation concerning the ornamentation of the exteriör 

 surface of the hypostoma of Ac. crenata. It is covered by a great nuinber 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 interiör something like a peculiar black spi- 

 culum rising from a bifid rootlet and confined within the wartlet and not extruding from 

 it. In a longitudinal section (Pl. I fig. (J) the spicula perforate the wartlets reaching 

 through their whole length. As in Calymmene these interiör pseudo-spicula are tubes, 

 filled with iron-pvrites. It is probable that these tubes were once benring setie and quite 

 as in Apus formed a fur of bristles. 



The strueture of the cephalic eyes (Pl. I f. 1 —2) is prisma tie, but the separate 

 prisma are rather short and broad. Their lower or interiör end is convex. The sepa- 

 rating lines between the single prisms are not always distinct. 



