638 



REPTILES 



^ip:i#3|i 



and aggregated {Malpolon, Dryophis, Sepedon*). In secretive, crepus- 

 cular, and some nocturnal colubrids (e.g., Lampropeltis, Rhinocheilus, 

 Arizona, Cemophora, Trimorphodon) the outer segments of types A 



and B are more or less enlarged and 

 tend toward a cylindrical form; but 

 the Type C element is even more rod- 

 like — leads the way, so to say (Fig. 

 183b). In another assemblage of noc- 

 turnal colubrids (Tarbophis, Dasy- 

 peltis, Leptodeira, Dipsadomorphus, 

 etc.) the Type C element has become 

 a perfect rod and contains rhodopsin, 

 but the other two elements have outer 

 segments which are no more than 

 intermediate between diurnal-colubrid 

 cones and full-fledged rods (Fig. 

 186). Moreover, the A and B types 

 in these forms are much elongated, 

 as if to put them in the background 

 not only topographically but physiol- 

 ogically. In nocturnal elapids (coral 

 snakes, kraits, etc.), simulations of 

 these various nocturnal colubrid pat- 

 terns occur, with the Type C element 

 again leading the trend toward sec- 

 ondary rod-hood. 



A very few nocturnal colubrids (e. 

 g., Hypsiglena, Phyllorhynchus [and 

 Lytorhynchus?]) have converted all 

 three cone types into massive cylin- 

 drical elements (Fig. ISV^) — perhaps 

 the most spectacular of transmuta- 

 tions, but actually a simple one since 

 these secondary rods lack rhodopsin, 



*These forms, when compared with the other members of their families, thus afford an 

 interesting comparison with the lizards: these snakes, and the lizards, have both striven for 

 maximal visual acuity by eliminating the poorest of three cone types — one which makes a 

 good rod, but a somehow poor cone. 



tThe single and double rods here have no definite mosaic arrangement in the retina, whereas 

 in geckoes the single and double rods form alternate rows running horizontally of the eyeball. 



Fig. 186 — Retina of Leptodeira annu- 



Idta, exemplifying the Tarbophis series 



of nocturnal colubrids. x 500. 



A- single cone (Type A element); B- 

 double cone (Type B element); C, C- 

 layer of rods (Type C elements); o- 

 outer nuclear layer; /- inner nuclear layer; 

 g- ganglion-cell layer. 



