BURROWING SNAKES 253 



is perfectly justified in doing so, because it can be shown from 

 various other tests that the great continents must have been 

 united by certain land connections, and that the present dis- 

 tribution of the Typhlopidae can be readily explained if we 

 assume that their dispersal took place at the same time. The 

 presence of Typhlops tenuis in Guatemala and Mexico, on that 

 account, points to an ancient union between these countries 

 arid to a survival in them of a relict fauna during a long course 

 of geological ages, in which the two countries may have been 

 separated. 



I suggested above (p. 131) that the rattlesnakes, belong- 

 ing to the genus Crotalus, were of early Tertiary origin. 

 The centre of dispersal lies in the south-western States of 

 North America, all the species except one being confined 

 to that continent. The black -tailed rattlesnake (Crotalus 

 terrificus) is the only one which, according to Mr. Boulenger,* 

 ranges from south-western North America to South America. 

 Whether the South American species is identical with the 

 North American one is a matter in which authorities disagree. 

 All concur, however, in the view that the genus Crotalus 

 reappears in South America in one or more forms, differing 

 but slightly from those inhabiting Mexico. And these South 

 American forms are not, as we might expect, confined to the 

 west coast, but have even penetrated to eastern Brazil, if 

 Mr. Boulenger's records are reliable. At the same time it 

 seems as if their range in South America was very discon- 

 tinuous. 



The fresh -water fish fauna of Central America is described 

 by Professor Eigenmann f as poor, the genera south of the 

 Isthmus of Tehuantepec being practically all South American. 

 The North American fauna is entirely distinct from that of 

 tropical America, the former niot having contributed a single 

 element to the fresh- water fish fauna of South America. It 

 is interesting to compare this view with those founded on 

 other fresh-water groups such as the mollusks and crus- 

 taceans. 



* Boulenger, G. A., " Catalogue of Snakes," Vol. III., p. 573. 

 t Eigenmann, C. H., "Freshwater Fishes of South America," pp. 521 

 522. 



