

8 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



of two rauscular impressions or points of attachment on the inside for tnuscles wbich 

 have attached the hypostoma to the walls of the glabella or the head. The real na- 

 ture of these so variable tubercular macuhe is, however, a quite different one, as we 

 bope to be able to demonstrate in describing all their varying shapes in several genera. 



The interiör surface of the hypostoma shows all the protuberances of the exteriör 

 surface (tubereular maculae, spines and granulations) as shallow pits, the exteriör grooves 

 being on the contrary elevated ridges. For the rest the interiör surface is smooth and 

 its margins are posteriorly covered with a more or less broad duplicature, from the la- 

 teral margins of which the posterior wings project. 



The manner in which Liljevall became aware of the interesting structural fea- 

 tures of which I am going to give an account, is as follows. We were about to describe 

 and delineate some new or not sufficiently known Upper Silurian trilobites from the island 

 of Gotland. In order to draw a specimen of Bronteus polyactin Angelin, which is rarely 

 found in the Wenlock and Ludlow strata of Gotland, Liljevall was prepariug and 

 cleaning its hypostoma. The shape of this hypostoma, as represented in pl. II tig. 22, 

 is clypeiform, tapering towards the posterior margin, which is regularly tongue shaped. 

 The anterior margin is faintly curved and at both sides elongated in a broad, pointed 

 wing. Its exteriör surface is divided in three transverse fields, defined through two shal- 

 low grooves, the anterior and lar ges t tield occupying more than the moiety, the me- 

 dian one is crescent-shaped and narrow, and the posterior one is nearly of the same size 

 and confluent with the lateral borders of the hypostoma, which near the median trans- 

 verse axis of the latter project in a blnnt angle on each side. The surface, for the rest 

 smooth, is covered by concentric, irregular terrace lines. On the interiör edge of the 

 upper groove two small tubercles are situated, one on each side doser to the lateral 

 margins than to the central axis. The size of their longitudinal axis amounts to 0,^6 mm. 

 in the shortest and to 1,07 mm. in the longest specimens. This axis is parallel with the 

 groove and consequentlv oblique to the longitudinal axis of the hypostoma. They are 

 oblongue or ellipsoid, their inferior apices bhuitly pointed or rounded, varying a little 

 as seen in the three specimens iigured, (Pl. II tigs. 23, 24, 25). For about two thirds their 

 surface is perfectly smooth or rather glossy, and the iower third is covered with a compact 

 accumulation of small granules. The extension of this peculiar accumulation varies, being 

 somewhat larger in some specimens, and the limit which runs oblique towards the glossy 

 part is more or less curved. Such a granule taken single is of the extremest minuteness, 

 nieasuring 0,055 millimeter at the highest. Their outline is perfectly circular and semiglobose. 



When Liljevall saw this group of granules he was at once struck with the 

 perfect, outward similarity between them and the facets of the compound eyes on the 

 upper surface of the head of this same species, and I fully concurred in the same view. 

 It now became of importance to ascertain whether the intimate structure also corresponded 

 in both, and microscopic sections were accordingly prepared. Although not so clear as in 

 other species of this genus a vertical section of a cephalic eye (Pl, II tig. 21) shows a 

 median row of black spöts which compared with other sections (Pl. II fig. 9) must be the 

 centres of the badly preserved lenses. This is confirmed by the horizontal section (Pl. 

 II Hg. 20) where the well known image of the polyedric facets appeared. In the same 



