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 wo 
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 not 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,” Yol. III., p. 573. 
[ Eigenmann, C. II., “ Freshwater Fishes of South America,” pp. 521 
—522. 
