Fig. 70.–Geographical distribution of Cryptodirous tortoises.
Fig. 71.–Geographical distribution of Pleurodirous tortoises.
The marine forms are naturally cosmopolitan, but the Testudinidae are likewise cosmopolitan, except in the Australian region. The Chelydridae, now restricted to North and Central America, occurred formerly also in Europe. The Pleurodira, in Mesozoic times plentiful in Europe, India, and North America, are now restricted to South America, Australia, and Africa; the Pelomedusidae to Africa, Madagascar, and South America; the Chelydidae to South America and Australia. In the latter country all the Chelonians belong to the Chelydidae. The Trionychoidea, occurring since the Cretaceous epoch in North America, in Early and Mid-Tertiary times in Europe, are now restricted to North America, Asia, and Africa. The country richest in Chelonians is America; North and Central America together possessing representatives of all the families except the Pleurodira, and these we know to have died out there. The Dermatemydidae, 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 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.
Fig. 72.–Geographical distribution of Trionychidae and Chelydidae.
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 retractile. 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 intertropical seas, but it is rare everywhere; least so perhaps in the Western Atlantic from Florida to Brazil and in the Indian Ocean.