350 REPTILES OF THE WORLD 



merely a "button" to represent the future rattle. The 

 first ring uncovered is larger than the button, and each 

 subsequent ring yet larger, until the reptile has at- 

 tained its full growth, when all of the segments pro- 

 duced are of uniform size. If a snake has a perfect 

 rattle — a button at the tip and successively larger seg- 

 ments — and three joints of the rattle are counted as 

 a year, a fair idea of the snake's age may be acquired* 

 If all the segments are of a uniform size, the reptile 

 has lost the segments of its youth — possibly many more 

 of them — through wear or accident, and no idea of the 

 serpent's age can be ascertained except that it is a per- 

 fectly mature specimen. The usual number of "rat- 

 tles" is from ten to twelve. 



Nineteen species of rattlesnakes are recognized. 

 The majority of them are restricted to the United 

 States and northern Mexico. The Southwest of the 

 former country is their headquarters, ten species oc- 

 curring in that region. Several species are restricted 

 to Mexico and Central America. But one occurs in 

 South America, where its range is extensive. 



Following is a list of the known species of rattle- 

 snakes : — 



Pigmy Rattlesnake, Sistrurus miliarius. S. E. U. S. 



Massasauga, Sistrurus catenatus. Central U. S. 



Edward's Rattlesnake, Sistrurus c. S. W. U. S. 



edwardsii. 



Mexican Pigmy Rattlesnake, Sistrurus ravus. Mexico. 



South American Rattlesnake, Crotalus durissus. Mex. to So. Am. 



Gray Rattlesnake, Crotalus pulvis. Cent. America. 



Black-Tailed Rattlesnake, Crotalus molossus. S. W. U. S.; Mex. 



Texas Rattlesnake, Crotalus atrox. Texas to Calif. 



Red Rattlesnake, Crotalus a. ruber. S. W. U. S. 



Diamond-Back Rattlesnake, Crotalus adamanteus. S. E. U. S. 



Timber Rattlesnake, Crotalus horridus. Eastern U. S* 



Pacific Rattlesnake, Crotalus oregonus. Western U. S. 



Prairie Rattlesnake, Crotalus confluentus. Central U. S. 



Tiger Rattlesnake, Crotalus tigris. S. W. U. S. 



