FISHES 315 



tened sphere with a relatively short antero-posterior diameter (Millot 

 and Carasso, 1955) ( Figs. 3()B-7). 



The cornea is flat and the sclera lined by a continuous cartilage, 

 thin (0-5 mm.) in front and thick (1-8 mm.) posteriorly where it encircles 

 the optic nerve. As with most fossil Crossopterygians and as in 

 C'hondrosteans. there is a pericorneal ring of calcified scleral plaques, 

 18 to 20 in number. The choroid is thin, the choriocapillaris being 

 particularly tenuous, and there is a well-formed cr3\stalline tapetum 

 which, being continued over the anterior surface of the iris, gives the 

 eye a brilliant metallic sheen. The ciliary zone is particularly rudimen- 

 tary, showing no I'adial folds nor any structure resembling a cam^^anula 



Fig. 365. — The CVxlacanth, Laiimeria cuALCMy.E. 

 (1,16 natural size) (after Giinther and Deckert). 



or other focusing apparatus. The lens, which approaches the cornea 

 leaving a very shallow anterior chamber, is almost spherical and large. 

 The retina is completely avascular and shows no area centralis. 

 As would be expected in the presence of the tapetum. the epithelium 

 is devoid of pigment. The visual cells are practically entirely 

 represented by long, thin rods ; cones are very rare and contain well- 

 defined, colourless oil-droplets, again recalling the corresponding 

 structures in the chondrostean eye. The general architecture of the 

 retina is poorly differentiated although Mliller's cells are particularly 

 numerous. Ganglion cells are few and their ratio to visual elements is 

 remarkably low. The eye is characterized by its great simplicity and 

 priniitiveness, presumably possessing a high sensitivity to light but 

 a rudimentary visual acuity. 



THE CHOXDROSTEAX EYE 



THE CHONDROSTEAXS are represented today only by the sturgeons and the 

 Polypterini. The sturgeons are a group of old-fashioned marine fi.shes which 

 ascend rivers to shed their spawn (caviare) in the Xorthern hemisphere, and 

 are today represented by Acipenser (Fig. 368) and a few related genera — Polyodon. 

 the spoonbill sturgeon of the Mississippi, Psephurus, an enormous fish of the 

 Yangtze-Kiang in China, and Scaphirhynchus, the shovel-nosed sturgeon of Polyodon 



