PALEONTOLOGIC CONTRIBUTIONS 1 33 



(p. 148, op. cit.) " an important visual organ until the lateral eyes, 

 which represent a later product, are fully developed. It may then 

 diminish in size and activity, but it rarely, if ever, wholly dis 1 - 

 appears." 



It appears from these citations that the parietal eye structure as 

 seen in Cryptolithus is not at all comparable to the larval ocelli 

 of insects or the ocelli of Limulus, the eurypterids and arachnids 

 in general, where the chitinous integument thickens into an exterior 

 lens, but that it agrees well with that of the phyllopods and other 

 crustaceans. It is quite probable that the thin black layer at the 

 base of the lenticular body is derived from the pigment of the 

 retina, and that there was but a cavity in front of the retina, filled 

 with either sea water or more probably some body fluid. This 

 cavity, acting as a lens, may have been present under all eye 

 tubercles of trilobites. We have no evidence of a pore that led 

 from it to the exterior in Cryptolithus tesselatus, but 

 have before us specimens of Asaphus expansus and 

 Isotelus gigas that show a pore, probably not accidental. 



The cases cited above (p. 131) of casts of the interiors of eye 

 tubercles of Asaphus expansus, raniceps, laevis- 

 simus. Basilic us kegelensis and Nileus arma- 

 dillo showing a distinct central pit and surrounding rim are 

 probably indicative of the presence of a similar fluid-filled , lens, 

 the interior wall of which produces now the depression. We also 

 have a finely preserved specimen of Ogygia desiderata 

 Barrande, exhibiting a finely preserved circular pit on top of the 

 tubercle. 



Aside from the direct evidence, from the structures just pre- 

 sented, of the ocular nature of the median tubercle of the glabella, 

 there is so much indirect evidence pointing to a visual function 

 that it alone would seem to be competent to allow a fair conclu- 

 sion. This evidence rests in the following facts : 



1 These tubercles are the sole prominences in the otherwise 

 entirely smooth carapaces of all the many genera of the Asaphidae, 

 in the Trinucleidae and other genera (see below) where they occur. 

 They must therefore have a function that requires prominence for 

 its performance, and they could not serve as spines as they are, as 

 a rule, too small for that purpose, although, in a few forms, p. e. 

 Megalaspis, they may have degenerated into spines as any other 

 organ is liable to do, and even in that case it is an open question 

 whether the median eye could not have been raised on the top 

 of a spine for better vision, the same as the lateral eyes have in 



