THE REPTILES 189 
thick-set and muscular, and provided with horny jaws 
entirely destitute of teeth, like those of the birds. The 
limbs also are usually short and thick and variously shaped, 
and adapted for aquatic or terrestrial locomotion. The 
number of vertebrae in the body and tail are relatively few, 
and the thick and heavy body is devoid of the elements of 
grace and agility of movement characteristic of the other 
reptiles. On the other hand, the former enjoy a freedom 
from the attacks of enemies not accorded to animals in 
general. 
At first sight the appearance of a turtle does not indi- 
cate a close relationship to the other reptiles, but a more 
.—Box-turtle (Terrapene carolina). 
careful examination, and especially of their development, 
discloses a remarkable resemblance. The head, tail, and 
limbs are essentially similar to those of the lizards, but in 
the trunk region peculiar modifications have taken place. 
The ribs at first separate, as in other animals, flatten 
greatly, and unite with a number of bones embedded in 
the skin, thus forming one great plate overlying the back 
of the animal. About the circumference of the shield 
other dermal or skin-bones are added, which increase the 
area of the carapace, and at the same time still others have 
