CLASSIFICATION. The Chelonia were at first divided according to modifications of the feet. Cope, recognizing the separability of Trionychoi dea, also, in 1870, emphasized a division based upon the mode of carrying the neck, one group (Pleurodira) bending it sidewise in withdrawing the head, and the other (Cryptodira) withdraw ing it in an S-shaped carve in a vertical plane. A more worthy division is Athecfc for the shell less Spargithrs or leatherbacks. and Theeophora for all other kinds. Boulenger's classification is as follows: ThePelomedusiche are a small family of African and South American turtles including the large and valuable ':Arran.' The family Chelydithe in cludes the curious matamata (q.v.). certain long necked Australian types (Chelodina), and the South American hydromedusa. The Caretto chelydithe are a family in which is set a strange small turtle of New Guinea, whose 'shell' is cov ered with soft skin instead of horny shields. Of the eryptodirous families, the Chelydrithr are rep resented by the various snapping or 'alligator' turtles (q.v.). and the Dermatemplida- by a few species of aquatic tortoises of Central America. The Cinosternithe are the family of the North American 'skunk' or 'stinkpot' turtles. (See Muss TORTOISE.) Platysternithe includes only a single Siamese water-tortoise. The Testudinidze, on the other hand, are the most populous of Chelonian families aud nearly cosmopolitan. The shell in this family is always covered with well developed horny shields, the plastron has nine bones, and the neck is completely retraetile.
About twenty genera with more than 110 species are now recognized; but the generic distinetions are not easily reeognized by external appearances. Ilere are classified the North American 'mud turtles' of the genus Chrysemys, whieh abound in all still waters, as do the 'pond-tortoises' (Emys) of Europe. the terrapins (q.v), and the box tortoises (Cistudo). The last gel their name from the fart that the plastron, which is united with the carapace by ligaments, is di vided into two movable lobes, connected by hinge permitting them to he lifted up against the overhanging carapace, and closing both ends of the shell perfectly after the animal has drawn within its defenses. The common box-tortoise of the United States Wishoto ('arolina) is entirely terrestrial, numerous everywhere and very inter esting in its habits. In this family, also, fall the various land-tortoises (q.v.) of the type-genus Testudo. Most of the foregoing are terrestrial or fresh-water forms, while the remainder are marine and have the limbs modified into swim ming paddles or 'flippers.' The family Che lonithe contains many great extinct forms. from the Cretaceous to recent times, and a few existing species, of which the green turtle, hawksbill, and loggerhead (qq.v.) are representatives. The order Trion•ehoidea contains only the family Trionychida., which have no rigid plates on the flat carapace, but a soft leathery skin, and the plastron imperfect. See SOFT-SHELLED TURTLE.