AMERICAN TURTLES 133 



geographical interest. Four species Oiccur in North America, 

 three of them being limited in distribution to the eastern 

 States. Only one, viz. : Clemmys marmorata, is quite con- 

 fined to the rivers and ponds west of the Cascade and Sierra 

 Nevada Mountains in Oregon and California. In Central 

 and South America the genus is unknown, but in eastern 

 Asia we find four species. One of them inhabits Japan, 

 another the island of Hainan, and a couple of them China. 

 Far away in western Asia we meet with another species in 

 Persia and Mesopotamia, spreading across Asia Minor to 

 Greece and Turkey, while finally Clemmys leprosa lives in 

 Spain, Portugal and north and western Africa as far as Sene- 

 gambia. The genus is thus quite confined to the northern 

 hemisphere. We might suppose that the ancestor of the 

 American Clemmys insculpta had spread northward from 

 eastern Asia in Pliocene times and, after crossing the Bering 

 Strait land bridge, had invaded Alaska and Canada, ulti- 

 mately reaching the eastern States of America in that 

 manner, and subsequently becoming extinct in the vast tract 

 of country which now separates the Chinese from the eastern 

 American species. The south-western Clemmys marmorata 

 would thus have been the latest development of the genus 

 in America. Such a supposition seems unlikely, owing 

 to the unsuitahility of the western States for sucfh 

 turtles. The few that now inhabit the west are rather to be 

 regarded as ancient survivals of a long-distant age when 

 the climatic conditions were much, more favourable for them 

 than at present. The earliest member of the genus indeed 

 (C. morrisiae), is found in a western Eocene deposit (Bridger 

 beds) , and Dr. Hay * expresses the opinion that North 

 America is probably the original home of Clemmys. If so, 

 the genus could not possibly have spread to Asia, and thence 

 to western Europe in Pliocene times. The existing centres 

 of distribution must have become established during much 

 more remote geological periods. Anyhow, although the 

 majority of the American species of Clemmys are now con- 

 fined to the east, we have ample palaeontological evidence 

 of its having formerly lived in the west. 



* Hay, 0. P., " Fossil Turtles of North America," p. 290. 



