244 WATER REPTILES OF THE PAST AND PRESENT 



nearly all of the costal plates and had the neurals and marginals 

 reduced. It is urged that some of these early marine forms, after 

 they had practically lost the ordinary bones of the carapace, for 

 some reason or other found a bony shell again necessary for their 

 welfare. Possibly they had become littoral in habit; possibly they 

 again became subject to new and dangerous enemies in their 

 unprotected condition, notwithstanding their great size; perhaps 

 the zeuglodons were among their enemies. Now. as we have seen, 

 an animal never takes a back track and recovers a thing it has 

 once lost. It was impossible for the ancestors of the leather-back 

 again to acquire an orthodox shell, and they forthwith proceeded to 

 acquire quite another kind that would serve the same purpose. 



Possibly the truth lies between the variant views, in the theory 

 recently expressed by Versluys: *'The shell of tortoises and turtles 

 is formed by a combination of two layers of dermal ossifications, a 

 thecal layer and a more superficial epithecal layer, the latter 

 generally represented by the marginals only. The leather-back 

 is a member of the Cryptodira, and is allied to the other marine 

 turtles. The problem of the origin of the aberrant shell of the 

 leather-back seems to find its solution in the h^-pothesis that it is 

 a secondary proliferation of the marginals and such other epithecal 

 elements as were present in its thecophorous ancestors." 



In other words, Versluys believes that Hay's and Wieland's 

 views of the primitive double layer of exoskeletal bones is essentially 

 correct, but that Dermochelys was derived from later forms in which 

 some of them, only, as in Archelon, had remained. Baur's conten- 

 tion that ''Dermochelys is not the least, but the most specialized 

 marine turtle" seems to have been fully justified. 



RIVER TURTLES. TRIONYCHOIDEA 



No reptile is more familiar or more exasperating to the river 

 fisherman than the turtle, variously known as the river, soft- 

 shelled, or mud turtle. It lives, often in great numbers, in most 

 of the rivers, ponds, and bayous of the interior east of the Rocky 

 Mountains, and especially in those of sluggish current and muddy 

 bottoms. It is voraciously carnivorous in habit, feeding upon the 

 smaller fish, mussels, and such other living food as it can capture. 

 With its long, sinuous neck and snake-like head, and soft, mottled 



