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 oliioensis. Its life was probably aquatic, and its 

 food consisted of vegetable substances, which it gnawed off with its powerful 

 incisors. 



The genus Palceomys has been admitted as an extinct genus of the old 

 Avorld, 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. 



Fam. 5. SciURiD^. 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 jDosterior 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 tlie 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, Pteromys volucella {pi. 113, figs. 12 and 13). An analogous species 

 is found in Poland, Eussia, 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 cat. 



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. hursarius), which inhabits deep burrows. Another is 

 from Hudson's Bay, and receives the common appellation of mole-shaped 

 sand rat (P. tcdpoidea). 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, father 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 

 672 



