C HEWN I A 227 



The shell in the more aquatic forms is depressed or flattened, 

 but in the terrestrial kinds may be as highly arched as in the true 

 land tortoises. The feet are adapted primarily for walking, but 

 nearly always have the toes webbed, and the digits are longer 

 than are those of the land tortoises. Only the two or three middle 

 toes have claws. Some species have developed hinges in the plas- 

 tron, whereby they may be completely closed up after the head 

 and legs are withdrawn within the shell. Most of the species are 

 carnivorous in habit, but a few, like the box tortoise, are strictly 

 vegetarian. 



Geologically the fresh-water tortoises have a not very ancient 

 history, going back no farther than do the land tortoises, that is, 

 to the beginning of the Cenozoic or Age of Mammals. Fully 

 fifty species are known from the Tertiary rocks of North America, 

 or more than three-fourths as many as are now living upon the 

 earth. 



The family at most can be said to be only amphibious in habit, 

 and has no striking aquatic adaptations, since the shell is well 

 developed and is covered with horny shields. The flattened shell 

 of the more aquatic forms is characteristic, as is also the greater 

 degree of webbing between the toes. 



LAND TORTOISES 



Perhaps the last of the more noteworthy specializations of the 

 Chelonia, and indeed among the last of the more important speciali- 

 zations of the Reptilia, are the upland tortoises, of which the com- 

 mon " gopher" of the southern states is almost the only remnant 

 in North America. They formed a part of the great hegira of 

 forest and marsh animals to the open prairies, away from the low- 

 lands and water which the turtles had inhabited almost exclu- 

 sively for millions of years. 



They began their career, Dr. Hay thinks, at about the begin- 

 ning of the Cenozoic, that is, with the great development of the 

 mammals, and reached the maximum of their development in the 

 Miocene; and they have been on the decline ever since. In the 

 Northern Hemisphere, at least, the slowly cooling climate through- 

 out the Eocene, and a decided decrease in moisture, brought about 



