170 S. R. DETWILER 



over except in the work of Heinemann and Krause, American 

 species have not been investigated. Of those mentioned by these 

 authors only one genus is represented, viz., Sceloporus, that 

 has been studied by me, and this is a Mexican species. The 

 need therefore for some account of the retina of the ordinary 

 American species of tortoises and lizards is still urgent, and since 

 the anatomical features of the visual cells have not yet been 

 described, a short account of these will not be out of place. 



To begin with the tortoises we find that in the retina of the 

 three species examined there are no rods. The cones are of 

 two sorts, single and double. The single cone is the more 

 numerous type of visual cell, and they are all similar in the 

 possession of an outer and inner segment, in the latter of which 

 is found in all cases an oil drop, an ellipsoid and a paraboloid. 

 In form and size, however, these single cones present individual 

 variations, on the basis of w^hich we may say that there are two 

 kinds, the first of which is considerably broader than the other, 

 but only a little longer. With this increase in size there is found 

 a slightly larger paraboloid and oil drop (fig. 6). 



The double- cones, of which there is only on« kind, are much 

 fewer in number than are the single cones. They are composed 

 of a principal and of an accessory part, there being no twin 

 cones. The principal cone has a very long narrow myoid, a 

 long ellipsoid and an oil drop, there being no paraboloid. The 

 accessory, which is much broader and shorter than the principal 

 cone, has the typical short myoid of a single cone, a paraboloid, 

 a granular ellipsoid but no oil drop (fig. 6). 



From preparations of fresh retinae it was found that the colors 

 of the oil drops were those which have been usually described 

 for the tortoise retina, namely red, orange, pale yellow and blue 

 green. The red are the largest and the most numerous. 



Krause ('93) describes single and double cones in the retina 

 of Emys europaea, there being two varieties of each. The first 

 kind of single cone is similar to that which we have just de- 

 scribed, the second is much broader and has only a coarse granu- 

 lar ellipsoid, but with no oil drop or paraboloid. Concerning 

 the double cones of Emys the first variety, which is extremely 



