682 



THE EYE IN EVOLUTION 



Trachysaurus 



The binocular fields of amphibians have not been thoroughly 

 explored, but particularly in Anurans it must be of considerable extent 

 (Schneider, 1957). 



The binocular fields of reptiles have been extensively studied 

 by Kahmann (1932) who found that they were more constant than in 

 Fishes : the average extent is between 20° and 30° with extremes at 

 14° in the lizard, Trachysaurus, and at 46° in the exceptional tree- 

 snake, Dryophis. 



Among the Chelonians, as elsewhere in the vertebrate phylum, the 

 extent of the binocular field varies with the habits of the animal ; 



Ftgs. 823 AND 824. 



BINOCULAR FIELD 

 ■30°-^' 



^^//VOARE^^° 



The Binocular Fields of Birds. 



BINOCULAR FIELD 

 60= 



Fig. 823. — The pigeon. Showing a small 

 anterior binocular field, large (pano- 

 ramic) uniocular areas and a small 

 blind area behind. 



•"^D ARE^ 



Fig. 824. — The owl. Showing the large 

 binocular field, small uniocular areas, 

 and a large blind area behind, charac- 

 teristic of a predator. 



Clemmys 



Iguana 



the smallest is seen in the placid herbivorous tortoise, Testudo (18°), the 

 more active terrapin, Clemmys, has a field of 34°, while the snapping 

 marine turtle, Chelydra, which is an activu predator of small fishes, has 

 a binocular field of 38° (Kahmann, 1933). 



Those Crocodilians which have been investigated have been found 

 to have a binocular field averaging 25° (alligator, 24° ; cayman, 26°). 



Lizards show much the same range as turtles. The smaller types 

 retain a wide panoramic field for protective purposes so that the avail- 

 able binocular range is low — Trachysaurus, 14° ; Anguis fragilis, 16° ; 

 Lacerta and Iguana, 18° (Fig. 819) ; while the larger and more militant 

 types, safe in their strength, enhance their aggressiveness by improved 

 binocularity {Zonurus giganteus, 22° ; Varamis, 32°). 



Snakes show a considerable variation in their binocular fields from 

 20° to 46°. Among representatives of the great central family of 



