TWIN CONES; OPHIDIAN DOUBLE CONES 61 



lengthen as one. These structures indicate that the makeup of the com- 

 mon double cone is worth imitating for some reason; and we shall see 

 shortly that the snakes have also discovered this for themselves. But, 

 until the distribution of these peculiar elements is better known and has 

 been related to teleostean taxonomy, there remains the possibility that 

 some of them are derivatives of holostean double cones (Fig. 24a) which 

 have never quite equalized their two members, rather than a secondary 

 departure of twin cones in the direction of double ones. 



Like the double cones of other classes, the twin cones of the teleosts 

 appear to be related to diurnal activity. Wunder has shown that they 

 are most numerous in surface fishes, less and less common in fishes 

 which habitually swim at greater and greater depths. Thus they seem 

 somehow to be associated with vision in bright light, though apparently 

 not with sharp vision since they are excluded from teleost foveae. More 

 than that cannot be said about them in the light of present knowledge. 



Ophidian Double Cones — The double cones of snakes are quite 

 unique. Though all lizards have double elements of the standard type 

 (Fig. 25a), the primitive snakes of the boa family have only single cones 

 of one kind, together with rods (Fig. 69b, p. 167). In the big central 

 family of snakes, the Colubridae, the standard retina contains only cones 

 of three types. One of these (Type A) is a large single cone and is 

 abundant. Another (Type C) is a small single cone which occurs always 

 in small numbers and is entirely lacking in the retinae whose resolving 

 power is highest. 



The Type B, double, cone (Fig. 24d) bears no resemblance to double 

 cones outside the snakes. Its chief member is bulky, and is identical 

 with the Type A single cone. The accessory is extremely slender and is 

 fused with the chief cone throughout the length of the inner segment. 

 The accessory nucleus is often displaced laterally in the outer nuclear 

 layer; and applied to it is an organelle, the paranuclear body, which 

 occurs only in ophidian double visual cells. Snake cones have no oil- 

 droplets or paraboloids, and the ellipsoid usually fails to stain with acid 

 fuchsin. The inversion of size-relationship of chief and accessory, the 

 paranuclear body, the absence of a paraboloid, and the extensive fusion 

 of the inner segments set the ophidian double cone off so sharply from all 

 others that even if it were present in the Boidae one could feel certain that 

 it was originated de novo within the snake group, and represents the 

 second — at least — separate invention of a double cone by vertebrates. 



