26 THE STORY OF REPTILE LIFE. 



hungry enemy must either pass on to other 

 game, or sit down and endeavour to take his 

 prize by siege ! As these creatures are able to 

 live for long periods without food, siege tactics 

 -are not likely to succeed. 



Before we leave the subject of the shell we 

 must briefly comment on the remarkable cara- 

 pace and plastron of the rare Leathery Turtle 

 {Dermochelys) of the West Atlantic and Indian 

 Oceans the largest of all living Chelonians. x 



Like that of the Tortoises we have already 

 examined, the shell is of dermal origin. That is 

 to say, it is made up of bony growths developed 

 in the outer skin ; but here the resemblance ceases. 

 For whereas in the Tortoises and Turtles gener- 

 ally these bony plates are symmetrically disposed, 

 and, in the carapace sink down on to and become 

 inseparably fused with the skeleton, in the 

 Leathery Turtle they take the form of innumer- 

 able small plates* interlocked one with another 

 to form a mosaic. The carapace of this turtle 

 is furthermore peculiar in that it remains per- 

 manently distinct from the skeleton, so that, 

 when removed the ribs and vertebrae are re- 

 vealed, muscle-covered, as distinct as in other 

 reptiles. The horny shields which cover the 

 shell generally in other Chelonians are here con- 

 spicuous by their absence; instead, the shell is 

 covered with a smooth, leathery skin hence the 

 name, " Leathery Turtle." 



The fundamental differences between the shell 

 of the Leathery Turtle and that of other Chelo- 

 nians has an important bearing upon the ques- 

 tion of the origin of the two groups. The 



