468 ZOOLOGY. 
An animal nearly allied to the beaver existed in North America during 
the tertiary epoch, but far surpassing in bulk those of the present time. 
Its remains were discovered in the State of Ohio, and described under 
the name of Castoroides ohioénsis. Its life was probably aquatic, and its 
food consisted of vegetable substances, which it gnawed off with its powerful 
incisors. ! 
The genus Paleomys has been admitted as an extinct genus of the old 
world, and has left the remains of one species (P. castoroides) in the 
tertiary deposits of Germany. Its dentition comes near that of the beavers, 
but sufficiently different in some peculiarities of structure to permit the 
establishment of a new genus. Per 
Fam. 5. Scturtp#. The characters of this family consist in having 
simple molars with tuberculous crowns, five above and four below on each 
side ; the lower incisors very much compressed. ‘The toes are long, armed 
with sharp claws, four on the anterior and five on the posterior feet; the 
anterior thumb is very short. The tail is long and tufted. Some are -pro- 
vided with cheek pouches. In others the skin of the sides is extended 
between the anterior and posterior limbs. We distinguish three kinds of 
squirrels: the true squirrels, the ground squirrels, and the flying squirrels. 
The genus Pteromys (flying squirrels) is easily distinguished from any 
other of the same family by the expansion of the skin between the hind and 
fore legs, by means of which they are enabled to support themselves for a 
moment in the air, and to make very great leaps. The American flying 
squirrel, Pieromys volucella (pl. 118, figs. 12 and 18). An analogous species 
is found in Poland, Russia, and Siberia. Besides, there are three more 
species in North America inhabiting the north and west. There is one 
found in the Indian Archipelago that is nearly of the size of a eat. 
The genus Pseudostoma possesses four prismatic molars above and below 
and on each side, the first of which is double, the others simple. The upper 
incisors are furrowed with a double groove in front. There are five toes 
to each foot. They have very short legs and very deep pouches which 
open externally, enlarging the sides of the head and neck. Four species 
are known to exist in North America. One is the Canada hamster or 
pouched rat (P. bursarius), which inhabits deep burrows. Another 1s 
from Hudson’s Bay, and receives the common appellation of mole-shaped 
sand rat (P. talpoidea). A third species is found in the northern regions, 
_and the fourth in Florida. 
The genus Aplodontia is destitute of cheek pouches. Its body is thick 
and short, clothed with fur like that of the musk-rat, but neither so long nor 
_so fine. The head is flat and broad; the nose a little arched, thick, and 
‘obtuse. There are five molars above and four below on each side. The 
limbs are robust and short, the feet moderately strong, with naked soles, all 
provided with five toes, rather short, but well separated. The thumb of 
the fore feet, however, is much shorter than the other toes. The claws, 
particularly the anterior ones, are very long, strong, much compressed, and 
but little curved. The tail is very short, and concealed by the fur of the 
hips. These animals form small societies, live in burrows, and feed, as 
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