REPTILES 



light-shunning turtles it is clear that they are physiologically rods, bear- 

 ing several signs of cone ancestry (Fig. 176b and Plate I). 



Rods are perhaps lacking in the foveate Amy da (Gillett figures only 

 cones), but this form's suborder, the Trionychoidea, is not primitive, 

 though characterized by a soft shell (so, secondarily) , We may be sure 

 that these droplet-free elements, serving originally as cones, were part of 

 the cotylosaurs' equipment, though their origin (presumably from drop- 

 let-bearing cones) cannot be traced 

 in any living vertebrates (Plate I) . 

 The other visual-cell types of the 

 turtle group are the same droplet- 

 bearing single and double cones 

 which we have already seen in the 

 amphibians and traced to the ar- 

 chaic Chondrostei (Fig. 176b; cj. 

 Fig. 174a, p. 599). 



g#; 



^ @ 



Fig. 176 — The chelonian retina and its visual cells. 



a, retina of common snapping turtle, Chelydra serpentina. x500. 



a- amacrine cells; d- double cone; g- ganglion cell; h- horizontal cells (ropy type); 'm- 

 Miiller fiber; n- bundle of nerve fibers; o- outer nuclear layer; p- pigment epithelium; 

 r- rod; s- single cone. 



b, single cone, double cone, and rod of Chelydra serpentina, x 1000. 



