HISTORY AND HABITS OP THE TESTUDINATA. 
NIX 
a solid obtuse stump, are considerably expanded, and connected by a mem¬ 
brane, so as to form powerful paddles. The claws, too, which in the Testu- 
dinidas are short and thick, are, in these, long, sharp and strong, to enable 
them to tear the animal prey on which they subsist. 
In this family there is a small group in which the characters indicate a mani¬ 
fest approximation to the Testudinidag. These are the Box Tortoises, in which 
we find the limbs more robust, the shell higher and less expanded, and, in short, 
all the characters intermediate between the terrestrial and the fluviatile forms. 
The sternum in these is divided into two or three portions, by one or two 
transverse sections: of these the anterior at least, and in many the anterior 
and posterior, are moveable ; being connected by means of a ligamentous 
or cartilaginous hinge; and are capable of more or less accurately closing the 
shell, when the animal is at rest. Of these the Terrapene clausa may be con¬ 
sidered as approaching most nearly to the Land Tortoises, and Terrapene euro- 
pcea as the most closely allied to the fresh-water group. 
The Trionychid^e exhibit perhaps the most remarkable and interesting 
structure in the whole order. They consist of those tortoises, inhabitants of 
the fresh water, which have no horny plates covering the shell, or any other 
part of the body. Instead of this more usual covering, they are invested with 
a strong coriaceous skin, which covers equally the dorsum and sternum, to the 
bones of which it is closely attached: there is also a free margin of the same 
leathery substance; and this circumstance, with the remarkable flatness and 
expansion of the animal, constitutes a form admirably adapted for concealment 
in the mud, at the bottoms of rivers and lakes; whilst the extraordinary length 
of the neck, and the powerful muscles by which it can be suddenly protruded, 
enable the animal to seize its prey by an instantaneous effort, without the 
necessity of always quitting its retreat. 
Another peculiarity of this group is, that the three outer toes only, of each 
foot, are furnished with claws. The bones of the sternum are separate, and 
the ribs are generally united only through a part of their length. 
The structure of the marine family, the Cheloniadje, exhibits the natatorial 
character in the highest degree, and at once points out these animals as per¬ 
manent inhabitants of the sea. The whole body is flat,—the shell so small as 
