Because the legs are no longer needed for the support or propulsion of the body in long-tailed air-breathers, their connection with the body becomes less and less firm, long before their entire disappearance. In animals using the legs for crawling or walking the bones of an arm and thigh are elongated, and the joints are always well formed, permitting varied, extensive, and firm movements. Just the reverse is the tendency in all those animals that propel themselves by the aid of the tail in the water, since here what is needed is broad, short limbs, not long and slender ones. Most reptiles have five digits on each hand or foot; the bones of the wrist and ankle are well formed, as in mammals, and the digits are elongate, with a very definite arrangement of the bones composing them, as already described, never exceeding five in any one finger or toe. In the paddles of water reptiles, as the limbs are usually called, the bones of the first segment, that is, the humerus and femur, are always greatly shortened in those having a propelling tail, and even in some with a short tail, such as the seals, and in a lesser degree in the sea-otters. On the other hand, in those animals which use the legs chiefly for direct propulsion these bones are elongated, as exemplified by the plesiosaurs and marine turtles. In all save the seals and their kind, and the otters, whose legs are used rather as sculls than as oars, the bones of the next segment, the radius and ulna of the front pair, the tibia and fibula of the hind pair, are always shortened, and one can tell the stage of aquatic adaptation, as exemplified, for instance, in the plesiosaurs and ichthyosaurs by the degree of shortening of these bones. Indeed, the first suggestion in any crawling animal of water habits is shown in the relative lengths of the epipodial bones, as these bones are called. Furthermore, cursorial or terrestrial habits are suggested by the relative size of the smaller bone of the leg, that on the little-toe side, the fibula. In birds, pterodactyls, and most running animals, it disappears in part or wholly. In swimming animals it tends to grow larger than the tibia, as will be conspicuously seen in the paddle of the mosasaurs.