26 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



The eyes of the Trilobites. 



If, as is probable, to judge by the conformity of their cornea with that of recent 

 crustacea, the trilobites like these were provided with crystalline cones beneath the corneal 

 lenses or facets, only the 1 ätter have been preserved in a fossil state. Although the cry- 

 stalline cones in consequence of their solid consistence might have been petrined as well 

 as the cornea, they must, imbedded as these tiny cones lie, entirely wrapped up in deli- 

 cate tissues, fall away and be löst, when the dissolution of the dead body had set in. 

 Consequently the curious appendages on the inferior side of the lenses in Dalmanites 

 vulgaris (Pl. III f. 50) or Phacops quadrilineata (Pl. V fig. 38) can noways be conside- 

 red as belonging to the original structure of the eye, apart from their great dissimilarity 

 with anything appertaining to the eyes of the Arthropoda. The cornea on the contrary 

 cohered with the integument of the body, and it has been well preserved in a great num- 

 ber of trilobites. 



I subdivide the trilobites in respect to the form of their eyes in the following 

 manner : 



I. Genera with compound eyes. 



1. With prismatic plano-convex cornea facets. 

 Acidaspis, as a transitional form to the next group. 

 Asaphus. 



Bumastus. 



Cyphaspis. 



Dysplanus. 



Encrinurus. 



Illienus. 



Megalaspis. 



Nilens. 



Phillipsia. 



Niobe. 



Proetus. 



Ptychopyge. 



Symphysurus. 



2. With round or biconvex transversally elongate lenses. 



Acerocarc. 

 Bronteus. 

 Chirurus. 

 Ctenopyge. 

 < 'yrtometopus. 



