626 



REPTILES 



numerous, long, and fine, but the pigment migrates so slightly that it 

 forms practically permanent sheaths around the individual cone outer 

 segments, as in diurnal snakes. 



The visual cells are always of two types (each varying, of course, in 

 size), which form a 'matching' single-double combination (Fig. 180). 



Fig. 180 — Visual-cell types in representative lizards, x 1000. 



a, single and double cones of a diurnal lizard, Crotaphytus collaris. 



b, single and double 'intermediate' elements of Xantusia river siana. 



c, single and double, completely transmuted rods of a gecko, Coleonyx variegatus. 



In the diurnal majority of families both of them are typical cones (Fig, 

 180a) with yellow oil-droplets, respectively homologous with the drop- 

 let-bearing single and double elements of all lower reptiles and the birds 

 and lower mammals — indeed, tracing their ancestry back to the chon- 

 drostean fishes (see Plate I). In the nocturnal 'leaf-footed'- or snake- 

 lizards (Pygopodidaj) of Australia, however, the oil-droplets have been 

 discarded and the outer segments somewhat enlarged to permit scotopic 



