With the exception of the land tortoises, all turtles from the beginning of their career as an order to the present time have been more or less at home in the water. In some, like the marine forms, the adaptation to aquatic life has produced marked changes in structure: in the loss of the horny dermal shields and in the loss of bone tissue; in the flattening of the shell, and in the development of the front legs into swimming flippers, with a loss of the claws. In the absence of a guiding tail, which is always small in the marine turtles, propulsion must of course be wholly by the aid of the limbs. As oar propellers the marine turtles show some of the peculiar characters of the plesiosaurs. With a like short and broad body, a more or less elongated and flexible neck, there could be no sinuosity of the body in swimming. As an oar-like organ the humerus became flattened, and its muscular attachments, as in the plesiosaurs, descended far down the shaft, giving greater mechanical advantage. Unlike all other aquatic vertebrates, the turtles never developed real hyperphalangy. Only in the river turtles is there a possibility of an increase in the bones of the fourth digit. To discuss in general the structure and habits of the living chelonians would extend this chapter to an undue length, and would add nothing to the many excellent works on natural history accessible to the student. We have therefore contented ourselves with a brief outline of the geological history of the order, with especial reference to their aquatic habits. SIDE-NECKED TURTLES. PLEURODIRA The suborder of Chelonia, generally known as the snake-necked or side-necked turtles or tortoises, comprises about forty living species, confined to South America, Africa save the northernmost part, Madagascar, New Guinea, and Australia. In Australia they are the only members of the order known—another instance of the peculiar isolation of the fauna of that region. In the past they lived in North America during Upper Cretaceous times, the earliest known forms of the group in its restricted sense, of which seven species are described by Hay. In Eocene times they are also known from Europe and Asia, from both of which regions they have long since disappeared.