148 REPTILES AND BIRDS. 
or Marsh Tortoises known, which are spread over all parts of the 
globe, but principally in warm and temperate regions. Such are the. 
Cistudo, Emydes, and Trionyx. 
The Elodians have none of the sluggishness of the Land 
Tortoises ; they swim with facility, and on land they walk with 
considerable rapidity. Their eggs are white, and nearly spherical, 
with a calcareous shell, and these are deposited in a hollow dug in 
the soil or sand, like the Land Tortoises, the place chosen being 
Fig. 34.—Mud Tortoise. 
generally situated on the banks of some secluded stream; the 
number of eggs increasing as the animal approaches maturity. 
The Elodians are divided into Cryptoderes and Pleuroderes: the 
former distinguished from the latter by the retractile power they 
possess of concealing their cylindrical neck, with its sheath of loose 
skin, under the middle of the carapace; the head, the width of 
which is nearly equal to the height at the occiput ; the eyes always 
lateral, and their orbit so large that the diameter of the cavity 
nearly equals a fourth of the whole cranium ; and the jaws, which are 
strong, sometimes trenchant, in others are dentated on the edge. 
In the larger number of species the anterior extremity of the upper 
