248 WATER REPTILES OF THE PAST AND PRESENT 



numbers of bones in the digits that mammals have, that is, two 

 phalanges in the thumb and big toe and three in each of the other 

 digits. The river turtles have a larger number in the fourth digit, 

 either four or five. It seems to be a law that evolution is irre- 

 versible, and if so could the river turtles have been descended from 

 forms with a less number of phalanges ? But, the skeleton of the 

 Trionychoidea resembles the more specialized turtles in so many 

 ways that one can hardly believe they were all accidental or parallel. 



We jnay then assume that at about the time that the ordinary 

 marsh turtles took to the sea to become marine, others took ad- 

 vantage of the fresh-water ponds and rivers, and in doing so, like 

 the marine turtles, lost their horny epidermal shields, and became 

 thinner in shape, thereby reducing the resistance to the water. 

 Instead, however, of reducing the costal plates over the ribs, they 

 retained them intact and complete for some reason or other, but 

 lost instead the marginal row of bones, unlike the marine turtles 

 which retained them even after they had lost nearly all of the costal 

 plates. Possibly also they regained additional bones in the fourth 

 digit, a sort of hyperphalangy like that of the more strictly aquatic 

 reptiles. Or, possibly, they may have descended from some branch 

 of the turtles which had not yet lost these bones, retaining them 

 because they were still serviceable for swimming. We know 

 nothing yet about the structure of the feet of the early turtles, and 

 it is possible that not all had acquired the reduced phalangeal 

 formula. 



In the development of aquatic habits the river turtles do not 

 show the same degree of specialization in the limbs that the strictly 

 marine forms do. The humerus (Fig. 131) is a slender bone, with 

 the tuberosities for the attachment of the muscles situated near 

 the proximal end. The radius and ulna are relatively short, and 

 the foot is long. The hind legs, as would be supposed, are less 

 highly specialized as swimming paddles, and are relatively smaller. 

 Nevertheless the Trionychoidea present an interesting type of 

 adaptation to water habits, both in body and in limbs. 



