The Pleurodira, as the term indicates, are easily distinguished from all other turtles by the way in which they withdraw the head within the shell. Instead of withdrawing it by an =S=-shaped flexure of the neck between the shoulder-blades, as do other turtles, these bend the neck laterally in a horizontal plane, bringing the head within the margins of the shell in front of one or the other foreleg; and the margins of the shell are produced here in an eave-like fashion for the greater protection of the head. In the structure of the shell, which is always fully developed into a box, these turtles do not differ very much from the Cryptodira, though there may be some extra bones in the plastron, as also in the skull. The nasal bones are always, the lacrimals sometimes, well developed; the latter never, the former rarely, found in other groups. The lower jaws articulate a little differently, and the external ear is always fully surrounded by bone. Very characteristic is the bony union of the pelvis with the plastron below, which never occurs in other turtles, unless it be the Amphichelydia. The side-necked turtles are all of fresh-water habit, similar to that of the fresh-water tortoises spoken of farther on. The neck is often very long and snake-like, which accounts for one of the names given to these turtles; because it is withdrawn into the shell sidewise, it has more distinctively ball-and-socket joints between the vertebrae, with distinct transverse processes for the attachment of the necessary side-moving muscles. The feet in all are more or less webbed and armed with strong claws. The largest and perhaps the best known of all living side-necked turtles is the giant Amazon turtle of South America, which sometimes has a shell nearly three feet in length. Its feet are broadly webbed, and the shell is rather flat in the adult; it is an excellent swimmer in the waters of the Orinoco and Amazon. Six or seven species of the genus to which it belongs are known, all of them South American except one that lives in Madagascar and one fossil found in the Eocene of India. This remarkable distribution is but one more of the many instances known in zoölogy and paleontology that seem to prove an early land connection between India and South America. Had the migration between the two continents occurred by way of Asia and Bering straits, as did that of hosts of mammals, one would certainly expect to find some evidence of it in the North American Tertiary rocks, which, so far, is lacking. CRYPTODIRA The chief families of the Cryptodira turtles are the Chelydridae, or snappers; the Emydidae, or marsh tortoises; the Testudinidae, or land tortoises; the Chelonidae, or sea-turtles; the Protostegidae, or ancient sea-turtles; and the Dermochelydidae, or leather-backs. Other doubtful or smaller groups, both living and extinct, may be omitted, or incidentally mentioned. SNAPPING TURTLES