MAMMALIA. 461 



Brazil. Finally, the tliird genus, cbaracterized by a tliick, short, and 

 depressed tail, covered above at the base with hairs and spines ; at the 

 apex, and on the under surface, with stiff bristles. The feet short and 

 broad ; four toes to the fore and five to the hind feet, armed with long and 

 curved claws. To this genus belongs the Canada porcupine {Erethizon 

 dorsatus\ which inhabits North America. That from the western coast 

 (California, Unalaschka, Sitka) has been described as a distinct species, but 

 there are still doubts entertained whether it is really distinct or not. Two 

 fossil species of this section are found in Brazil. 



The porcupines of the old world form two genera. The genus Hystrix 

 includes the common porcupine of Europe, H. cristatus {jjI. 113, fig. 4), 

 together with other species from different regions of Asia. The genus 

 Atherura, with its tail nearly as long as the body, contains two species, one 

 from Africa, the other from southern Asia. 



Remains of the porcupine have been found in the centre of Europe, and 

 about the Sivalic Mountains in Asia, but not yet characterized. 



The fresh water tertiary deposits of France have yielded other remains 

 which seem more intimately related to the new world porcupine than to 

 those of the old, and for which the genus TJierldomys was proposed to 

 include temporarily one species. 



Fam. 3. MuRiD^. This is the largest family of the rodents, and is com- 

 posed of animals of moderate size ; indeed, some of the smallest of the class 

 belong to it. The cutting teeth, two in each jaw, are awl-shaped in the 

 lower ; the molars are simple or compound, the upper shelving backwards, 

 the lower forwards ; the limbs are proportionate ; the tail scaly ; fur, with 

 scattered long hairs. The family may conveniently be subdivided into 

 eight sub-families. 



Suh-fam. 1. Saccomyina, is a somewhat doubtful or excentrical group, as 

 far as known at present, its affinities with the other sub-families having not 

 yet been made out fnlly. The animals which compose it, known as sand 

 and mole rats, are provided with cheek pouches which open externally, 

 four molar teeth, sometimes rootless and sometimes rooted. The tail, short 

 in some, is long in others. 



Here are referred the following genera : — Dipodomys^ Ilacrocolus, 

 Jleteromys, Saccornys, Perogyiathus, and Geomys. 



The genus Oeomys comprehends the largest number of species which 

 constitute the sub-family, and chiefly North American. The Columbia 

 sand rat {G. douglasii) is one of them. Its body is shaped like that of the 

 mole, and covered with soft, dense, velvety fur, of a uniform brown color. 

 It has large cheek pouches hanging down the sides of the head, the latter 

 being large and depressed, the nose obtuse, particularly when viewed in 

 profile. The tail is more than half the length of the body, round, tapering, 

 and obtuse, covered with hairs, particularly near its base. The legs are 

 short and thick. The claws are very sharp pointed, compressed, curved, 

 and about as long as their respective toes. The palm is naked, and its 

 posterior part is filled by a large, rounded, callous eminence. The hind 

 feet are a little more slender than the fore ones, and they are armed with 



665 



