18 VISIBLE STRUCTURE 



very powerful and thick shell, whilst in the Oleneides, which always present impressions, the 

 shell was decidedly thinner, and consequently became entirely lost. The few existing 

 positive facts, however, do not permit us to determine this point with certainty, and we must, 

 therefore, content ourselves with having pointed out the possibility of one or the other mode 

 of organization. 



SECTION VI. 



The number, position, and structure of the eyes can be ascertained with much more 

 certainty, and are therefore comparatively well known. There are, however, still many 

 deficiencies in the knowledge existing on these points, which is the more surprising since 

 we are enabled to make a perfect representation of them from actual observation. 



In all those Trilobites the eyes of which can be distinctly recognized, we see them in the 

 shape of more or less considerable prominences at the sides of the head, nearly on the centre 

 of the lateral portion of the shield, projecting from the latter. They are here seen as portions 

 of a spherical or parabolic surface, under a semicircular projection (the cover of the 

 eye, or of the horny covering of the head), which projection is formed by the sutura tempo- 

 ralis, (of which a description will be found subsequently,) and in reality they fill out a chasm 

 that is situated at this locality between the two opposite margins of the suture. The 

 eyes project in the shape of a half-cone, flattened on the upper part, if this vacuity is large ; 

 if small, it forms a lunate protuberance, which is so slight in some species that it scarcely rises 

 above the contiguous surface of the head. Such Trilobites have been considered as blind ; 

 and with regard to Oleums, which genus possesses the structure described, it is given as a 

 generic character. The eyes of the lUamiis are in the shape of a moderately arched, lunate 

 swelling ; in Ascqjhus, CaJijmene, and Phacops, they appear as more highly elevated tubercles 

 or hemispheres. Whilst the external surface of the eyes in the other genera is per- 

 fectly smooth, and even more so than the neighbouring horny covering, there appear in 

 PJiacops, instead of these, small hemispherical elevations distributed over the entire surface 

 in regular order, the small interstices shaping themselves into protuberantly swollen in- 

 cisures of the hemispheres. Owing to this, the eyes of the Trilobites ai'e usually represented 

 as being formed on two different types, assigning to the former a smooth, to the latter 

 a facetted cornea. This view, which is entertained by all fomner observers, I must consider 

 as decidedly incorrect : first, because there is not a single existing family of Articulata 

 in which the eyes are formed according to two different types; and, secondly, because 

 the character of the facettes in Plutcops is quite different from the mode of formation pre- 

 dominating among the Articulata with a facetted cornea. I am rather of opinion that all 

 Trilobites possessed compound eyes with a smooth cornea, and that the latter has merely 

 been lost in those genera in which facettes are perceived. In addition to the two reasons 

 mentioned, I am further justified in this assumption by the fact that the cornea of most 

 of the Trilobites is really smooth, and that the structure of the eyes of those species, 

 to which a facetted cornea is attributed, is in every respect such as it would be if their 

 eyes possessed a simple, smooth cornea, which was subsequently lost. This, therefore, 

 seems to be the proper place to explain more particularly the structure of the compound 

 eyes with a simple, smooth cornea. 



