358 BuLLiSTix 83 



VERTEBRATES 



First in importance of the animals to be considered herein are 

 the poisonous reptiles,— the snakes and the Gila Monster. When 

 poisonous snakes are mentioned, the average person in the South- 

 west will think at once of rattlesnakes, and probably only of rattle- 

 snakes, and we shall see that there is considerable justification in 

 fact for this. Rattlesnakes or "rattlers" are distinguished from all 

 other snakes on the globe by the possession of a peculiar appendage, 

 the rattle. This rattle, by the way, does not at all accurately indi- 

 cate the age of its bearer. Careful observation shows that each 

 shedding of the skin results in the formation of a new section of 

 the rattle, and that as many as three or four sheddings may occur 

 in a year. Division of the number of segments of a complete, un- 

 broken rattle by three will give more nearly the true age of the 

 reptile. 



SNAKES 



Rattlesnakes are found only in the New World, mainly in 

 North America above Panama, and more in the southwestern 

 United States than elsewhere. Hence Arizona is practically the 

 center of the chief rattlesnake inhabited area of the world. To put 

 this in another way, nineteen species of rattlers are known. Only 

 one of these is found south of Central America. Of the other 

 nineteen kinds, fourteen occur in the United States, and of these 

 Arizona has within its borders, according to the available data, 

 eleven. It is hardly within the main purpose of this bulletin to 

 discuss the different kinds in detail, yet doubtless the reader will 

 be interested in a brief resume of the kinds, sizes, and relative 

 abundance of these species, as well as suggestions as to their prob- 

 able distribution in the State. 



All rattlesnakes are divided into two principal groups: the 

 Pygmy rattlesnakes (Sistnirus), and the larger rattlesnakes {Cro- 

 ^alus). As the name indicates the Pygmy rattlesnakes are of small 

 size, seldom as much as a yard in length, and the rattles are small. 

 There are more kinds of the larger rattlesnakes than of the Pygmy 

 rattlesnakes, and since they are usually larger they are better 

 known. (Certain details of structure enable the specialist to dis- 

 tinguish between them regardless of size.) Only one variety of 

 the Pygmy rattlesnakes is known to occur in Arizona, Edward's 

 Massasauga, (Sisf runts cafenatus edzvardsii.) This is one of the 

 largest of its group, attaining occasionally to a yard or even more 



