ADAPTATIONS 125 



chelonian skeleton is remarkable for the circumstance that the 

 shoulder and pelvic girdles lie within — instead of without — the 

 ribs. Consequently the upper plates of the carapace lie directly 

 upon the spines of the vertebrae and the ribs, to which they 

 ultimately become welded. The marginal plates of the cara- 

 pace have, however, no subjacent stratum belonging to the 

 interna] skeleton. As regards the plastron, it will be sufficient 

 to state that while its three anterior elements respectively re- 

 present the clavicles (collar-bones) and inter-clavicle of other 

 vertebrates, its hind constituents appear to correspond with the 

 so-called abdominal ribs found on the ventral aspect of the body 

 in crocodiles, plesiosaurs, and tuateras. Evidently, then, 

 chelonians are descended from a group of reptiles furnished 

 with such a system of abdominal ribs, but beyond this we are 

 in the dark as regards their ancestry. 



As in crocodiles, the dermal bony armour of most tortoises 

 and turtles is overlain by a series of horny shields ; but there 

 is the important difference that whereas in the members of the 

 former group the epidermal horny shields correspond in size 

 and in number with the subjacent dermal bony plates, in the 

 latter group they do not so correspond. As a matter of fact, 

 although there is a general agreement in the plan of both the 

 horny shields and the bony plates, the former are fewer in 

 number and larger in size than the latter, and cover portions of 

 the series to which they do not pertain. It is consequently 

 believed that the two structures have been developed indepen- 

 dently of one another at different stages of the evolution of the 

 group, the horny plates being presumably the older. That is 

 to say, the ancestral chelonians are believed to have been pro- 

 tected by horny shields alone, and to have had no bony dermal 

 armour. The final stage of evolution, so far as the covering 

 of the body is concerned, is the loss of the horny shields, 

 which in the soft river-tortoises of the family Trionychidce 

 are replaced by a continuous leathery skin. 



Such are the general features of the chelonian shell. In the 

 luth, or leathery turtle (Dermochelys coriacea), now representing 

 the family Dermochelyidce, the shell is of a different type. It is 

 described by Dr. H. Gadow in the volume on reptiles in the 

 Cambridge Natural History in the following words : — 



" The dorsal and ventral halves are directly continuous, 



