2404 



TORTOISES, TURTLES, AND PLESIOSAURS 



THE LAND TORTOISES AND TERRAPINS 

 Family TESTUDINID^E 



The land tortoises, together with the greater number of the fresh-water 

 tortoises, or terrapins, of the Northern Hemisphere, as well as their southern 

 allies, collectively constitute one of several families belonging to the first great 

 group of the order. From the circumstance that all its 

 members are so constructed as to be able to withdraw 

 their heads within the margins of the shell by a bending 

 of the neck in an S-like manner in a vertical plane, the 

 group may be conveniently designated S-necked tortoises, 

 their scientific designation being Cryptodira. Since, how- 

 ever, the soft tortoises likewise retract their heads in a 

 similar manner, it is obvious that this characteristic alone 

 will not suffice to define the group, and it must accord- 

 ingly be supplemented by others. Although the degree 

 of ossification of the shell is very variable in the group, 

 the carapace and plastron being in some cases welded into 

 a complete box, and in other instances separate, yet, ex- 

 cept in the leathery turtle, there is invariably a complete 

 series of marginal bones, connected with the ribs; the 

 presence of the full series of marginals, together with 

 the S-like retraction of the neck, being sufficient to distin- 

 guish the group. A peculiarity in which the members of 

 the group differ from those x>f the next one, is to be 

 found in the circumstance that the bones of the pelvis 

 remain throughout life unconnected with the plastron; THE 

 while in the greater number of cases the latter, as shown 

 in the accompanying figure, comprises only six pairs of 

 horny shields, there being no intergular shield between 



the first pair, or gulars. The skull is characterized by the tympanic ring (/ in the 

 accompanying figure) having a notch in its hinder border, and also by the con- 



dyle on its quadrate bone fitting into 

 a hollow at the hinder end of the lower 

 jaw. This S-necked group includes the 

 marine turtles, and all the tortoises of 

 the Northern Hemisphere, with the ex- 

 ception of the soft river tortoises, and 

 thus comprises by far the greater num- 

 ber of the living representatives of the 

 entire order. Although well represented 

 SIDE VIEW OF THE SKULL OF A LAND TORTOISE in Africa and South America, the group 



HALF OF THE 

 PLASTRON OF THE CHAI- 

 BASSA TERRAPIN. 



WITH THE LOWER JAW REMOVED. 



is quite unknown in Australia. 



