igi6. No. lO. THE LOWER CAMBRIAN HOLMIA FAUNA. 65 



eye of Lininlits that I cannot avoid the conckision that thev are of the 

 same type". He thus approaches Packard's conclusion that the eyes of 

 the trilobites were built in the same manner as those of Liniiilits. whilst 

 L1NDSTRÖM considered that they were quite different from the eyes of the 

 latter type, and on the contrary- agreed with those of Isopothi. 



Two cranidia in my material have well preserved eyes. Both 

 belong to fullgrown individuals. One — the best preserved — is 

 seen reproduced in pi. \'II, figs. 1 — 3, and it can also be seen more 

 schematically in the text-figure 10. The surface of the eyes is elong- 

 ated and low, a little oblique and quite faind}' arched. Both ends 

 are rounded oft", the anterior one being a little pointed. The entire eye 

 is surrounded by a faint furrow. The surface itself is well preserved at 

 several places, and consists of quite fine, hexagonal, smooth facets. There 

 are no traces of the "fine, rounded, hexagonal openings", but on the con- 

 trary we find a cornua built as usual in Trilobites. 



As will be seen, we do not find here the structure of the ocular 

 membrane that Walcott described in the case oi the juvenile stage of 

 Olenellus Gilbcrti. 



In order to explain this disparity two possibilities are conceivable. 

 Either the eye in the quite xoung nepionic stages possesses a different 

 structure to that of mine specimens, or the eye in Walcott's specimen 

 must have been defective. I am very much inclined to agree with the 

 latter conclusion. 



Maculae of the H y pos tome. 



A well preserved hypostome in m}" material shows not only the 

 original form, but also several details in an excellent manner. Thus both 

 maculae are well preserved, (pi. \'I1, fig. 6). The}- are of marked oblong 

 form, rather broad above, and then suddenly diminish in breadth in the 

 lower portion. This lowest and pointed part has a markedly pitted 

 surface ; the sculpture of the upper portion is ver}- indistinct. The entire 

 macula is surrounded by a fine, raised rim. There is also a narrow 

 markedly sculptured zone Just above the macula. 



Articulation of the Thoracic Segments. 



As mentioned in the general description, we see on the posterior edge 

 of the dorsal furrow on all the thoracic segments a marked inscision, of 

 which Holm was quite well aware; what he did not see, however, was 

 that a small bud-shaped prolusion on the anterior edge of the following 

 segment fits into the inscision, and thereby produces a kind of articulation. 

 Vid.-Sdsk. Skrifter. I. M.-X. Kl. iqi6. No. lo. 5 



