38 
NATURAL HISTORY. 
£upper FLOOR. 
have the body covered with square imbedded shields, and 
fiave the tympanic bones sunk in the base of the solid 
hard bony skull; the vent is roundish, or linear and 
plaited, and the generative organs are usually simple. 
They comprise the Tortoises, ( Chelonians ,) Emydosauri , 
and Amphisb emails. 
The Tortoises ( Chelonians , Case 2) are peculiar 
for having all the muscles of the body placed within the 
thorax, out of the front and hinder part of which the 
head, limbs, and tail are exserted, as out of a case. This 
case is formed of the dilated back bone, ribs, and breast 
bones. When the animal is first born, these bones are 
separate from each other, as in the other Vertebrata, but 
as they grow they gradually enlarge, and approximate, and 
at length are united together by a toothed suture, until 
they form a very substantial protection to the body. They 
are divided into families, according to the form of their 
feet, which vary with the habits of the animals. 
Those which live on land ( Testudinidae, Case 2) have 
club-shaped feet, and very solid, convex shells, with a single 
marginal caudal plate. These live chiefly on vegetable 
substances, and bury themselves in the ground during 
the winter : their eggs, some of which are exhibited, are 
of a globular form. These animals are often used as food., 
especially by sailors, who procure them from the Galla- 
pagos, where they have been lately introduced, and have 
increased very rapidly. 
The Tortoises which live in water have a more depressed 
shell; their feet are expanded and webbed between the 
toes, and the latter are furnished with sharp claws. They 
are divided into three groups. 
The first of which, the Terrapins, ( Emydce , Case 2,) 
have twelve horny plates on the breast-bone or sternum, 
and withdraw their head and neck between the shells. 
These are much used as food by the Americans and the 
natives of India. Amongst these the case contains a mon¬ 
strosity with two heads. 
The second group ( Chelydm ) have an additional horny 
plate on the front of the sternum, and bend the neck back 
under the side margin of the shell. 
The third group ( Trionycidce ) are peculiar from having 
the shell covered with a naked skin, and only three toes 
