30 
NATURAL HISTORY. [UPPER FLOOR. 
much used as food by the Americans. Amongst these is a 
monstrosity with two heads. 
Those of the second group ( Chelys ) have an additional 
horny plate on the front of the sternum, and bend the neck 
back under the side margin of the shell; as the Radiated 
Chelys (Hydraspis radiolata). 
The fresh-water Tortoises of the third group (Trionyx), 
are peculiar from having the shell covered with a naked 
skin, and only three toes of each foot provided with claws. 
They, like the rest, are strictly carnivorous, and eat their 
food in the water. Many large specimens of this genus 
are found in the East Indies, and are frequently seen prey¬ 
ing on human bodies as they float down the Ganges. 
There are in this Case young specimens of two of these 
species, the Hurum Trionyx (Trionyx hurum), and the 
Punctated Trionyx (Trionyx punctatus ). The latter is 
peculiar for the leg, when drawn up, being covered by a 
moveable flap placed on the sides of the chest. 
The Marine Tortoises or Turtles ( Cheloniae ), are distin¬ 
guished by their feet being compressed and fin-shaped; 
they live principally on fuci and sea-weeds. These are 
the Tortoises most commonly used as food, and great 
quantities of one species (Test, Midas) are annually 
brought to England for that purpose. The horny 
plates of the Imbricated Turtle (Test, imbricata) afford the 
best sort of tortoise-shell. Over the mantel-piece is a 
large specimen of the Coriaceous Turtle, caught on the 
coast of Dorsetshire. 
The Alligators are peculiar to America, and are dis¬ 
tinguished from the Crocodiles, which are found both in 
the Old and New w r orld, by their feet having the toes free, 
that is, unconnected by webs, and by the canine teeth of the 
lower jaw being received in pits in the upper; whereas, in 
the Crocodiles, they are received in a notch on its margin. 
The Gavials agree with the Crocodiles in regard to their 
teeth, but differ by the muzzle being very long and slender. 
Specimens of all the three genera are in the Collection. 
The upper jaw of these animals is generally, but erroneously, 
said to be moveable ; the mistake having arisen from the 
lower jaw being much produced posteriorly. 
Case 3 contains those Saurian Reptiles which have the 
tongue long and deeply forked; namely, the Monitors of 
