2456 



TORTOISES, TURTLES, AND PLESIOSAURS 



. S - necked and Side - necked 

 groups, and which may have 

 been the ancestral stock of 

 both the latter. All have 

 eleven bones in the plastron, 

 owing to the presence of 

 mesoplastrals, and an inter- 

 gular shield, but the pelvis 

 may or may not be connected 

 with the plastron. In the 

 first of the genera named, the 

 mesoplastral bones extend 

 right across the shell to meet 

 in the middle line, and one 

 of the bones of the pelvis 

 articulates to a smooth oval 

 facet on the plastron. On 

 the other hand, in the second 

 genus, the mesoplastral bones 

 are incomplete, as in the ex- 

 isting greaved tortoises, and 

 there is no union between the 



pelvis and the plastron. Since it is probable that the plastron of the Chelonians 

 has originated from a system of abdominal ribs similar to those of the tuateras 

 (Chapter VI. ), it is interesting to notice that these generalized tortoises had a larger 

 number of plastral elements than are to be found in the majority of the existing 

 representatives of the order. 



IMPERFECT CARAPACE OF WIDE-SHIELDED WEAI.DEN 

 TORTOISE. 



THE SOFT TORTOISES 

 Family TRIONTCHID^ 



The last group of the order comprises the soft river tortoises, now confined to 

 the warmer regions of Asia, Africa, and North America, but which, during the 

 middle portion of the Tertiary period, appear to have been extremely abundant in 

 the rivers of England and other parts of Europe. The whole of these tortoises are 

 included in a single family which forms a group of equivalent value to the S-necked 

 and vSide-necked sections; and it is not a little remarkable that while in the greater 

 part of their organization they approximate to the former group, in certain features 

 connected with the skull they come nearer to the latter. The most striking pecul- 

 iarity of the soft tortoises is to be found in the nature of their shells, which are 

 covered with a raised sculpture of variable form, and are quite devoid of horny 

 shields. The lower shell, or plastron, is always very imperfectly ossified, and com- 

 pletely separate from the carapace, while the carapace never has a complete series 



