REPTILIA, BATRACHIA, AND PISCES. 117 
of fish occurs that is found in the United States. Crocodiles and Tortoises, like many 
families of Anura, are ancient and widely distributed. Snakes and Lizards are more 
modern, and their dispersal seems to have been recent and rapid; in these groups 
there is no marked difference between the nearctic and neotropical regions; no 
orders or families that show much regard for the boundary; the differences are 
for the most part generic or specific, and are mainly due to the sudden change 
from the dry plateau of the southern United States and Mexico to the tropical 
forests of Central America. . 
Of the groups dealt with in this chapter, only the Fishes give clear indications as to 
the past, and from them we infer a Cretaceous connection between South America and 
Africa that may have persisted into the Eocene, and was succeeded by a long isolation 
of South America and in the Miocene by its connection with North America, which 
has been since the Eocene until quite recently more or less continuously connected 
with Asia across the Bering Sea. | 
