MAMMALIA. 461 
Brazil. Finally, the third genus; characterized by a thick, 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 (Hrethizon 
dorsatus), which inhabits North America. That from the western coast 
(California, Unalaschka, Sitka) has been described as a distinet 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 poreupines of the old world form two genera. The genus /T/ystrix 
includes the common porcupine of Kurope, fH. cristatus (pl. 118, jig. 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 Hurope, 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 Theridomys was proposed to 
imelude temporarily one species. 
Fam. 8. Murip&. 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. 
Sub-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 fully. 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, Macrocolus, 
Heteromys, Saccomys, Perognathus, and Geomys. 
The genus Geomys comprehends the largest number of species which 
constitute the sub-family, and chiefly North American. The Columbia 
sand rat (G. douglasi’) 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 
