172 S. R. DETWILER 



is much thicker, with a large paraboloid, a granular ellipsoid, 

 and no oil drop (fig. 7). 



Krause ('93) holds that Lacerta agilis has four kinds of visual 

 elements, one of which is a rod, the other three being cones. 

 The second kind of single cone is distinguished from the first 

 by having a thicker inner segment and lacking an oil drop, 

 although there is present an ellipsoid and a paraboloid. Double 

 cones come principally from a combination of a principal cone 

 with an oil drop and an accessory cone without an oil drop, 

 though he also found double cones, both parts of which contained 

 an oil drop. Heinemann ('77) finds two kinds of cones, those 

 with oil drops and those without. The cones with oil drops 

 show all gradations from voluminous cones with large lens- 

 shaped bodies to very thin ones with very narrow inner segments 

 in which there is no lens-shaped body. These latter unite with 

 cones without oil drops to form double cones. 



Greeff ('00, p. 117) figures a single and a double cone for the 

 retina of a lizard. The single cone is quite similar to the narrow 

 type found in Sceloporus (fig. 7), the only difference being that 

 the niyoid is comparatively shorter and thicker. The second or 

 broad type of single cone, which is the predominating type of 

 single cone in Sceloporus, he does not figure at all. The double 

 cone is entirely similar to the one found by me. 



In passing, it is interesting to note that in the tortoise retina 

 the external nuclear layer consists of two rows of nuclei. Of 

 these only those nuclei in the row immediately internal to the 

 external limiting membrane are cone nuclei, the other row being 

 bipolar nuclei which connect the cone nuclei with those of the 

 inner nuclear layer. The cone nuclei are in general larger and 

 more oval, with the long axis in the same line as that of the 

 cones. In Sceloporus, however and Cnemidophorus (the sand 

 lizard) the external nuclear layer consists of only one row. 

 Chievitz ('89, p. 146) considers the second row, which he found 

 in both Emys and Lacerta, to be the nuclei of supporting cells. 

 Another point of interest is the presence of a fovea and large 

 papilla in the lizard retina, but neither of these in the tortoises, 

 though there is a small area centralis. Heinemann f'77, p. 425) 



