140 CLASS MAMMALIA. 



We now come to the genus Gerboa. This genus ap- 

 proximates considerably to the Rats properly so called, by 

 a great number of characters of internal organization, but 

 is sufficiently distinguished by the shortness of the anterior 

 limbs, and the length of the hinder extremities, or to speak 

 more correctly, of the hinder metatarsi, and by the tail, 

 which is covered with long hairs at its extremities. 



As to external conformation, the Gerboas exhibit some 

 relations with the Kanguroos. The form of the body is the 

 same in general. The hinder limbs are likewise five or six 

 times stronger than the fore. In both genera the tail is 

 very long ; the ears elongated, and pointed, and the eyes very 

 large and round. But though the Kanguroos have so many 

 traits of external conformation similar to the Gerboas, 

 they are infinitely removed from them in most important 

 points, such as the organs of generation, ventral pouch, fyc. 

 Erxleben was therefore decidedly wrong when he classed the 

 Kanguroos with the Gerboas, under the name Jaculus Gi~ 

 ganteus. 



The Gerboas have the same teeth as the Rats, that is, 

 they have two incisors in each jaw ; and the lower, instead 

 of being flat and cut scissors-like, as the upper, on the con- 

 trary, are conic and pointed. The molars are generally six 

 in number, three on each side. They are slightly sloped. 

 There is sometimes an additional one in the upper jaw. 



In the Gerboas the cheek-bones are very prominent, which 

 gives a singular and flatted form to the front part of the 

 head. The muzzle is short, large, and obtuse. A considerable 

 number of stiff" hairs extend on each side, and form long 

 mustachios. The nose is naked, cartilaginous, and in one spe- 

 cies rather complicated. The ears are long and pointed ; the 

 eyes large, and placed altogether on the sides of the head. 



The body is a little elongated, larger behind than before, 

 and well covered with soft and silken hairs. The fore-feet 

 are very short and feeble. They have four or five toes ac- 



