9 8 



TORTOISES AND TURTLES. 



approaching this conformation is to be met with among living representatives of 

 the order. 



Generalised Certain extinct tortoises, such as Pleurostemum from the 



Cneionians. Purbeck Oolite of Swanage, and Baena of the Eocene rocks of the 



United States, indicate the existence of an extremely generalised group of the 



order Amphichelydia, presenting many characters common to the existing 



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 

 existing 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 generalised 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 WEALDEN TORTOISE. 



The Soft-Tortoises. 



Family Trionychid^e. 



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

 greater part of their organisation they approximate to the former group, in certain 

 features connected with the skull they come nearer to the latter. The most 

 striking peculiarity of the soft-tortoises is to be found in the nature of their shells, 



