ADAPTATION OF LAND REPTILES TO LIFE IN WATER 69 



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 adapta- 

 tion, as exemplified, for instance, in the plesiosaurs and ichthyosaurs 

 by the degree of shortening of these bones. Indeed, the first sug- 

 gestion 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. 



The bones of the wrist change in two ways: by becoming 

 cartilaginous, as in whales and salamanders, or by becoming more 

 firmly ossified and more closely united, as in the plesiosaurs. The 

 digits always are elongated, often extraordinarily so, either by the 

 elongation of individual bones or phalanges, or by the development 

 of new bones. These new bones, when they occur, are new growths, 

 not the reproduction of the old elements of fishes, and there may 

 be as many as twenty such new elements or phalanges in a single 

 digit. There is one marked exception among reptiles to this 

 hyperphalangy, as the increased number of phalanges is called, and 

 that is the turtles. As we have seen, in the elongation of the neck 



