THE VERTEBRATES 207 



fifteen hundred species being known in the world, but it 

 is represented in the United States by comparatively few 

 kinds. Specimens of some species of the common swift 

 are obtainable almost anywhere in the United States. They 

 may be looked for in woods, along fences, and especially 

 on warm rocks. In certain regions the glass-snake or 

 joint-snake is common. This lizard, popularly thought 

 to be a snake, has no external limbs, and its tail is so brittle, 

 the vertebrae composing it being very fragile, that part of 

 it may break off at the slightest blow. In time a new tail 

 is regenerated. It ■ lives in the central and northern part 



Fig. 105. The Gila monster, Heloderma horridum. (One fourth natural 

 size; photograph from life by J. O. Snyder.) 



of the United States, and burrows in dry places. In the 

 western part of the country horned toads are common, 

 about ten different species being known. These are liz- 

 ards with shortened and depressed body and well-developed 

 legs. The body is covered with protective spiny protuber- 

 ances, and in individual color and pattern resembles closely 

 the soil, rocks, and cactuses among which the particular 

 horned toad lives. All the species of horned toads are 

 viviparous, seven or eight young being born alive at a time. 

 In New Mexico, Arizona, and northern Mexico the only 

 existing poisonous lizard, the Gila monster (fig. 105), is 

 found. This is a heavy, deep-black, orange-mottled lizard 

 about sixteen inches long. There is much variance of belief 

 among people regarding the Gila monster, but recent ex- 

 periments have proved the poisonous nature of the animal. 

 The poison, which is secreted by the glands in the lower 



