TORTOISES AND TURTLES. 



land. Even these purely marine forms are, however, in the habit of coming 

 on shore for the purpose of depositing their eggs. It is important to re- 

 member that in ordinary language the term turtle is often applied to many 

 of the fresh-water forms, although it is preferable to restrict it to the marine 

 species. Whereas all the other members of the order lay hard -shelled 

 eggs, those of the turtles are soft-shelled. In the matter of diet, Chelonians 

 present as much diversity as in their other habits, some kinds being ex- 

 clusively carnivorous, while others feed on vegetable substances. 



Sub-Order J. Cryptodira. 



The members of this group are primarily characterised by their power of 

 withdrawing the head within the margins of the shell by an S-like bending of 

 the neck in a vertical plane ; but as the same feature is to be found in I ho 

 third group of the order, supplemental characteristics are necessary to define 

 the present sub-order. With the exception of the abnormal leathery turtle, 

 the shell of all the forms no matter what be its degree of development is 

 furnished with a complete set of marginal bones, some of which are articu- 

 lated to the extremities of the ribs. Accordingly, the Cryptodira may be de- 

 lined as Chelonians which retract the head by means of a vertical S-like flexure 

 of the neck, and either possess a complete set of marginal bones attached to 

 the ribs, or have the limbs converted into nippers. These are, however, by no 

 means the sole distinctive features of the group. An important structural 

 peculiarity is to be found in the circumstance that none of the bones of the pel- 

 vic girdle have any attachment to the lower shell or plastron. Then, again, as a 

 general rule, the latter bears only six pairs of horny shields, an intergular shield 

 being but very rarely developed, If the dried skull be examined, it will be found 

 that the ring of bone surrounding the aperture of the ear technically termed 

 the tympanic ring is interrupted by a distinct notch in its hinder border ; 

 while the quadrate-bone articulates with the hinder end of the lower jaw by 

 means of a convex knob or condyle, received into a pit in the latter. 



With the exception of the soft tortoises, this group includes all the land 

 and fresh-water Chelonians of the Northern Hemisphere, as well as the 

 marine turtles and the leathery turtle. It is, however, by no means wanting 

 in the Southern Hemisphere, as there are numerous forms both of terres- 

 trial and fresh- water habits in Africa and South America, although none 

 are known from Australasia. There are such a large number of genera to 

 say nothing of species included in this group, that limitations of space 

 permit of reference only to some of the more important types. 



This extensive family group is probably best known in the shape of the 

 common Grecian tortoise and European pond-tortoise, both of which are com- 

 monly exhibited for sale in London shops, although neither 

 are native denizens of the British Islands, In this family Land-Tortoises 

 are also included the giant tortoises of the Mascarene and and Terrapins. 

 Galapagos Islands. There are no marine forms; but while Family 

 some are terrestrial, others are more or less amphibious, Testudinidce. 

 spending a largo portion of their lives in fresh water. With- 

 out going into abstruse and wearisome anatomical details, it is by no means 

 easy to give a concise definition of the family. It may be observed, however, 

 that while the head is capable of complete retraction within the margin oi 

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