THE OPHIDIAN RETINA 637 



the osculant Xenopeltis comes to histological examination. The 'stand- 

 ard' colubrid retina is pure-cone, with three types of elements, only one 

 of which (Type C) has the structure and staining behavior of the boid 

 cone; and this type is present only in small numbers at best (Fig. 185a). 

 It was probably the progenitor of the abundant Type A single cone 

 (see Plate I), which in turn somehow gave rise to the unique ophidian 

 double cone (Type B; see Fig, 24, p. 59). 



Fig. 185^ — Representative visual cells of diurnal and secretive colubrids. xlOOO. 



(Here, and in the illustrations on the next two pages, the hoinologous elements are labelled 

 with capital letters designating their type; see text). 



a, the three cone types present in all diurnal colubrids and elapids (except where Type C 

 has been discarded; see text); drawn from Natrix natrix. 



b, visual cells of the scarlet snake, Cemophora coccinea, exemplifying a number of secretive, 

 crepuscular, and semi-nocturnal colubrids. Note enlargement of the outer segments, and 

 tendency of Type C toward a rod-like form (c/. C in Fig. 187). 



It is interesting — in fact, fascinating — to note that with their pro- 

 duction of this diurnal colubrid arrangement, the snakes had at last 

 struggled back to a pattern which strikingly simulates that of the archaic 

 reptiles : single and double elements which match in structure, plus an 

 'odd man' in the form of the Type C cone. And, the snakes have 

 wandered off into nocturnality by various pathways, for the Type C 

 cone, like the droplet-free cone of the cotylosaurs and eosuchians, has 

 shown itself to make a better rod than a cone: 



In those diurnal colubrids and elapids which exhibit high visual acuity, 

 the Type C cone has been eliminated and types A and B are slenderized 



