IX GEOGRAPHICAL DISTRIBUTION ATHECAE 333 



Cinosternidae, and Chelydridae are now restricted to the Nearctic 

 sub-region (including Central America). Poorest in genera and 

 species, all of them Chelydidae, is the Australian region, where no 

 fossils of other families have yet been discovered. Europe, with its 



B TRIONYCHIDAE. CHELYDIDAE. 



FIG. 72. Geographical distribution of Trionychidae and Chelydidae. 



few Testudinidae, does not come into consideration ; Asia has at 

 least Testudinidae and Trionychidae, and in addition the solitary 

 Platysternum in Indo-China, representative of a family whose 

 affinities with the Chelydridae again proclaim the validity of the 

 Periarctic region. 



Order I. ATHECAE. 



The vertebrae and ribs are not fused with, but are free from, 

 the carapace, which consists of numerous small polygonal plates 

 and is covered with leathery skin without any epidermal shields. 

 The limbs are transformed into paddles. The neck is not retrac- 

 tile. Marine. 



Fam. Sphargidae. Sphargis s. Dermatochelys coriacea, the 

 Leathery Turtle or Luth, is the only recent species and is the 

 largest of all recent Chelonians. The biggest specimen in the 

 national collection is about six feet and a half long, from the nose 

 to the end of the shell, which latter is about four feet long ; such 

 a specimen may weigh half a ton. Agassiz, however, says that 

 he has seen some " weighing over a ton." -The general colour is 

 dark brown, either uniform or with yellow spots. The Leathery 

 Turtle has a wide distribution, ranging over all the inter- 

 tropical seas, but it is rare everywhere ; least so perhaps in the 



