48 TORTOISES AND TURTLES. 



it never has an intergular shield in front. The limbs are adapted more or less 

 completely for walking, and are never modified into paddles ; while the head is 

 capable of complete retraction within the margins of the shell. A very important 

 structural feature in the shell is that the nuchal, or unpaired median bone in the 

 front of the carapace, does not send back processes underlying the marginal bones 

 of the same ; while in the tail each vertebra has a cup in front of its body or 

 lower portion, and a ball behind. None of the members of the family are marine 

 but while some are inhabitants of the land, others are more or less exclusively 

 dwellers in fresh water. There are, moreover, equally important differences in 

 regard to their food, all the land forms being herbivorous, while of those frequent- 

 ing the water some subsist on vegetable, and others on animal substances. 



By far the most numerously represented genus of the whole 

 family is the one including the true or land -tortoises, of which 

 there are rather more than forty existing species (counting a few that have 

 been exterminated within the historic period). These tortoises, of which a few 

 are more or less aquatic in their habits, have the upper and lower portions oi 

 the shell completely welded together, the former being frequently very convex 

 and much vaulted ; while the top of the head is covered with large horny 

 shields. The limbs, which are entirely adapted for walking, are of a club- 

 like form, and are covered with large horny scales or tubercles; their toes 

 being unwebbed and furnished with strong, claw-like nails. The tail is always 

 short, its proportionate length not being greater in the young than in the adult. 

 More important characters are, however, furnished by the bony shell and skull, to 

 observe the former of which it is of course necessary that the horny shields 

 should be stripped off. In a shell thus treated it will be seen that the unpaired 

 median neural bones of the carapace are relatively short and wide, and so 

 arranged that a four-sided one is interposed between two that are octagonal, 

 although in some cases they are mostly hexagonal; while the costal or lateral 

 plates are alternately narrow above and broad below. Moreover, the line dividing 

 the costal horny shields from the marginals usually corresponds with the suture 

 between the corresponding bones of the carapace, whereas in the other members 

 of the family one is above the other ; while a further peculiarity of most species of 

 the genus is that there is but a single caudal horny plate at the hinder end of the 

 carapace. In the skull the palate is provided with one or two ridges on each side ; 

 while the hinder aperture of the nostrils is situated on the line of the eyes. It 

 may be mentioned here that, as in the majority of the representatives of the order, 

 the form of the shell differs considerably in the two sexes ; the male having the 

 central region of the plastron deeply concave, while in the females it is flat 

 or slightly convex. 



True tortoises are distributed over Southern Europe and Asia, the whole of 

 Africa, the southern portions of North America, and South America (inclusive 

 of the Galapagos Islands). They are strictly herbivorous in their diet ; and certain 

 species, now confined to oceanic islands, attain gigantic dimensions, and are by far 

 the largest representatives of the family. The species inhabiting colder regions 

 hibernate during the inclement season by burrowing in the ground, whereas those 

 found in more genial climates are active throughout the year. All the species 





