510 University of California Publications in Zoology [Vol. 12 



Glancing over the characters which distinguish desert reptiles we 

 find that great power of locomotion is possessed by all but a few of the 

 forms. The latter include the rattlesnakes and "Gila monsters," 

 which have special means of protection, and the chuckwalla, gecko and 

 night lizard, which seem to be restricted to habitats where they find 

 immediately accessible retreats. The desert whip-tailed lizard, the 

 gridiron-tailed lizard, the desert iguana, the ocellated sand lizard and 

 the red racer all live in the open and forage at great distances from 

 cover. The leopard lizard, probably the greatest runner of all, preys 

 on other swift saurians, and the Bailey collared lizard does likewise. 

 The latter, however, lives among the rocks where cover is always 

 quickly available, and its agility is doubtless associated with its car- 

 nivorous habits. In many of the swifter lizards the tail is held above 

 the ground and used as a counter-balance while running. 



Loose sand characterizes one of the typical desert habitats. Many 

 remarkable adaptations in the desert reptiles are developed or pre- 

 served by this kind of an environment. In the burrowing snake, 

 Sonora occipitalis, in the gridiron-tailed lizard, in the ocellated sand 

 lizard, and in the desert horned-toad, the rostrum is specially devel- 

 oped, protruding beyond the mouth. These reptiles swim into the sand 

 by lateral movements of the head, and the lizards mentioned do not 

 employ the fore feet in digging as does the Cnemidophorus, or whip- 

 tail group. The eyelids of the burrowing lizards are fringed and meet 

 tightly along thickened edges and the nostrils are collapsible. In the 

 sand-loving desert horned-toad the ear opening is usually covered by 

 the granular integument, while in other species of the same genus 

 it is not. 



The toes on both front and hind feet of the most characteristic 

 American sand lizard, Uma notata, are broadened by a fringe of 

 elongate scales, and in this respect parallel certain species native to 

 desert regions in other parts of the world. 



Keen vision and alertness are attributes of most desert lizards. In 

 Dipsosaurus, Callisaurus, and Uma, alert lizards inhabiting open 

 plains with sparse vegetation, the head is held aloft and the body is 

 propped up on the fore limbs when the creature gazes about (see pi. 22, 

 fig. 7). 



The tail-dropping faculty of the species of lizards here discussed 

 varies from almost perfect autotomy in the night lizard to entire 

 absence of this function in the chuckwalla. The latter uses its tail as 

 an organ of defense. Cnemidophorus, Uma, Callisaurus. Uta, Scelo- 



