ORDER RODENTIA. 141 



cording to the species. The thumb or interior toe*, where 

 it exists, is very short, rounded at its extremity, and pro- 

 vided with an obtuse nail. The other toes are long and 

 armed with crooked nails. 



The hind-feet are as disproportioned as those of the Kan* 

 guroos, being four or five times longer than the fore-feet. 

 They are terminated by five or six toes, according to the spe- 

 cies, which are armed with short, but large and obtuse 

 claws. The three middle toes are always supported by a 

 single metatarsian bone, terminated by as many articulary 

 pulleys. This arrangement is analogous to what is ob- 

 served in the Ruminantia and Birds. When there are but 

 three toes, there is in all but one metatarsian bone. When 

 there are five, there are three bones in the metatarsus, one 

 of which alone is very strong, the lateral bones being very 

 short and slender. 



Ancient and modern naturalists have both been mistaken 

 respecting the walk of the Gerboa. They have all imagined 

 that these quadrupeds walked on their hind feet only, never 

 employing the fore-feet for that purpose. From this error 

 the genus was named dipus, two-legged. 



M. Olivier, from a consideration of the structure of this 

 animal's body, puts an end to this error; clearly proving 

 that the Gerboa was incapable of sustaining itself for any 

 length of time upon the tarsi. It usually walks on its four 

 feet; but when frightened from any cause, it endeavours to 

 escape by means of prodigious leaps, which it executes with 

 equal force and activity. When these animals are about to 

 leap, they raise their body upon the extremity of their hind 

 toes, and support themselves upon their tail. Their fore- 

 feet are so closely attached to their breast, that they are 

 scarcely visible. Having taken their spring they leap, and 

 fall upon their four feet ; then they elevate themselves again 

 with so much celerity, that it almost appears that they are 

 constantly in an erect posture. 



Vol, III. M 



