2 20 WATER REPTILES OF THE PAST AND PRESENT 



have thought the turtles and plesiosaurs were allied. The sacrum 

 is composed of two vertebrae only, and the pelvis of the usual 

 three bones, the ilium, the ischium, and the pubis, all covered over 

 by the shell. 



In every known turtle the neck is composed invariably of eight 

 vertebrae, but they are peculiar in many respects. In the earliest 

 known turtles the neck vertebrae were, as would be supposed, 

 biconcave, but they soon became very variable in all; in each neck 



some are biconcave, some 

 biconvex, some opisthocoe- 

 lous, and some procoelous. 

 And Dr. Hay tells us that 

 the neck has increased in 

 length in the later forms. 

 The skull also is very 

 peculiar in that it has 

 some very primitive char- 

 acters and others very 

 aberrant. The temporal 

 roof, as has been said, has 

 no holes through it, though 

 it is often reduced by the 

 emargination of the 

 borders, whether from be- 

 low or behind, until in 

 some the whole temporal 

 region is exposed, and not 

 at all covered over. There 

 is no parietal foramen, so 

 constantly present in all the early reptiles and in the lizards and 

 the tuatera of modern times. There are no teeth or vestiges of 

 teeth, but the jaws have usually a homy cutting edge, which seems 

 to be quite as serviceable; in the river turtle the Ups are fleshy. 

 There is no transverse or transpalatine bone. There is a single 

 vomer only, not paired as in other reptiles, whence comes the 

 doubtful theory that the vomers of other reptiles are not the real 

 vomers originally so named in mammals, and hence often called 



Fig. 114. — Pelvis of Chelone, from below: 

 pu, pubis: is, ischium; il, ilium (in acetabulum). 



