VERTEBRATA: MAMMALIA. 757 



Beaver shows extraordinary ingenuity in these and similar 

 operations; but there can be equally little doubt as to the 

 greatly-exaggerated stories which have been set afloat in this 

 connection. The tail is greatly flattened and scaly, and the 

 animal gives the alarm by striking it upon the water. The 

 Beaver is hunted chiefly for the sake of the skin, but also for 

 the substance known as castoreum. This is a fatty substance, 

 secreted by peculiar glands, and employed as a therapeutic 

 agent. 



Fam. 9. Sa#omyd&. This family comprises the so-called 

 Pouched Rats and Gophers of North America, all of which 

 have large external cheek-pouches. Some of them (Geomys 

 and Thomomys) have the fore -feet greatly developed, and 

 adapted for burrowing ; whilst the so-called " Kangaroo-rats " 

 (Dipodomys] have very long hind-legs, and the fore-limbs are 

 not specially developed. The Gophers (Geomys, &c.) possess 

 a pair of cheek-pouches, which are hairy inside, and open out- 

 side the mouth, their use being to carry provender. The best- 

 known species is the common Pocket - gopher (Geomys bur- 

 saritts) of the Mississippi valley and Canada. 



Fam. 10. Spalacidce. Nearly related to the American Go- 

 phers are the Mole-rats (Spalax] of the Old World. These 

 have a thick body, short legs, the tail rudimentary or absent, 

 the molars rooted, and the feet five-toed. The Mole-rats are 

 burrowing animals, in which the eyes are very small, and may 

 be covered over by the skin, so as to be functionally useless. 

 They live upon vegetable food, unlike the Mole, and some of 

 them lay up a winter store. Georychus and Bathyergus are 

 African forms of the group. 



Fam. ii. Muridce. The next family of Rodents is that of 

 the Muridce, comprising the Rats, Mice, and Lemmings. In 

 this family the tail is long, always thinly haired, sometimes 

 naked and scaly. The lower incisors are narrow and pointed, 

 and there are complete clavicles. The hind-feet are furnished 

 with five toes, the fore-feet with four, together with a rudi- 

 mentary pollex. The family comprises over three hundred 

 living species, distributed over the whole world, except the 

 islands of the Pacific, some of the species (such as the Brown 

 Rat and the common Mouse) being similarly cosmopolitan in 

 their range. 



The Rats (Mus rattus and Mus decumanus], the common 

 Mouse (Mus musculus), the Field-mouse (Mus sylvaticus), and 

 the Harvest-mouse (Mus messorius) are all well-known examples 

 of this family, and are too familiar to require any description. 

 The three first are also common in North America, though not 



