No. 3-] ACUTE VISION IN VERTEBRATES. 481 



preserved to make certain the presence of an area. It is only 

 visible in sections. 



In the lizard an area has been described in every case, and a 

 fovea in all but two, which are doubtful. The only lizard which 

 I have examined, the horned toad (Phrynosoma cornutum), pos- 

 sesses a deep and sharp fovea, situated on a broad band-like 

 area. The fovea is situated about the center of the retina, just 

 above the entrance of the optic nerve, which is marked by a 

 slender conical pecten. The band-like area is broadest in the 

 region of the fovea, and extends horizontally across the retina. 

 A dark line extends about i mm. to either side from the fovea 

 and gives the appearance of a trough-like fovea, as seen in the 

 tern, but cross sections reveal no depression. The band-like 

 area gradually becomes indistinct some distance from the ora 

 serrata (PI. XXVII, Fig. 15, and PI. XXX, Figs. 56, 57). 



In the turtles only an area has been found which is oval or 

 round in shape, and lies about the center of the retina, just 

 above the nerve entrance. It is not visible to the naked eye, 

 and in sections is noticed rather as a closer arrangement of 

 the cells than as a thickening. PL XXX, Fig. 61, represents 

 a section through the area of Chelydra serpentina. A repre- 

 sentation of the entire retinal section would be necessary to 

 show any difference in thickness. In an injected specimen of 

 Chelopus insculptus, a short and seemingly rudimentary blood- 

 vessel was noticed (PI. XXVII, Fig. 16) which seemed to be 

 an approach to a retinal circulation. In the other eye it was 

 not so long but similarly located. 



In the crocodiles Chievitz has described and pictured a band- 

 like area and shallow trough-like fovea which extend horizontally 

 through the entire retina. I have not been able to examine any 

 species of this order. 



Amphibians. 



The presence of an area and absence of a fovea seems to be 

 the rule. Hulke and Chievitz, however, have described a shal- 

 low fovea in Bufo vulgaris and Bufo calamitia, though in some 

 cases it is wanting. I have found a band-like area in Bufo 

 lentiginosus, Rana virescens and catesbiana. It is not visible 



