222 



WATER REPTILES OF THE PAST AND PRESENT 



possessed that number. All other reptiles, save those phylogeneti- 

 cally aUied to the primitive mammals, that is, the Theriodontia 

 and their allies, have normally the phalangeal formula 2, 3, 4, 5, 4. 

 It was partly because of this similarity of the numbers of toe bones 

 that the turtles have been classed in the great group of reptiles 

 that includes the ancestors of the mammals; that is, under this 

 theory, the turtles would enjoy a nearer relationship to the mam- 

 mals and to man himself than any other hving reptiles! But this 

 classification has been shown to be quite artificial. 



Fig. 117. — Limbs of Colpochelys, a recent sea-turtle: H, humerus; R, radius; 

 U, ulna; r, radiale; i, intermediiun; u, ulnare; p, pisiform; c, centrale; T, tibia; 

 F, fibula; a, astragalus; m, fifth metatarsal. (From Wieland.) 



From what has been said, it will be surmised that the Chelonia 

 represent in themselves one of the primary subdivisions of the class 

 Reptilia, and that, unlike most others, the order has enjoyed a most 

 remarkable longevity. And doubtless they are one of the primary 

 branches of the reptiUan stock, which has remained distinct since 

 Permian times at least, if not since Carboniferous, isolated and 

 remarkably homogeneous, giving off no branches which departed 

 far from the main stock, and on the whole leading a singularly 

 placid existence for ten or more million years. 



In most textbooks the order Chelonia is divided into three 

 suborders, the Pleurodira, the Cryptodira, and the Trionychoidea. 



