REPTILES 



365 



The eyes of diurnal lizards contain a central area at the posterior 

 pole wherein the cones are longer and thinner than in the peripheral 

 retina ; in addition, in diurnal varieties a central fovea is present 

 wherein the cones are closely packed, long and filamentous (Fig. 434). 

 The fovea is very striking in such forms as the American horned 

 " toad," Phrynosoma (Detwiler and Laurens, 1920 ; Ochoterena, 1949), 

 but is seen in its most fully developed form in the chameleon. The 



PJirynosoma 



Fig. 43-1. — The Remarkably Well-formed Fovea of the Gippslaxd Water- 



DRAGOX, PHTSKJyATHCS (O'Dav). 



ch, the thick choroid ; ?•. the remarkably well-formed retina ; s, scleral 

 cartilage ; sc, sclera ; v, \isual cell^;. 



remarkable fovea of this animal wherein the cones are longer (100^), 

 their concentration higher (756,000/sq. mm.), and the pit deei3er than 

 in the fovea of man, has long excited admiration (H. ^Nliiller, 1861-72 ; 

 Chievitz, 1889 ; A Vails, 1942 ; Detwiler, 1943 ; Rochon-Duvigneaud, 

 1943 ; and others). In nocturnal species, on the other hand, only a 

 trace of a foveal pit ma}' be observed {Xanfusia) or it may be entirely 

 lacking {Heloderma, and usually in the geckos). In some geckos a shal- 

 low temporal fovea exists {Gonatodes ftiscus, Sphoerodacfylus argus, S. 

 parkeri, Underwood, 1951)^; while in certam arboreally active species 

 of the diurnal lizard, Anolis, in addition to the deep central fovea, a 



^ Gonatodes has a pure-cone retina, SphcKrodnctijlus argus has visual elements 

 intermediate between rods and cones, .S. parkeri has a pure-rod retina and, incidentally, 

 a pure-rod fovea. 



