120 MAMMALS OF THE PACIFIC WORLD 



of the famous Norwegian lemming which during its years of 

 abundance makes great migrations, often ending in what looks 

 like mass suicide, plunging into the sea and swimming out until 

 drowned. The migrations of our lemmings are less well known 

 and less extreme, but they have a similar cycle of alternating 

 abundance and scarcity. The total length of this vole is about 

 five inches, of which the tail is about one-half inch, excluding 

 the hairs. 



The Collared Lemming {Dicrostonyx) may also occur on 

 the Aleutians nearest the mainland. It is colored gray, darker 

 or black down the middle of the back, and has a reddish collar. 

 In winter it turns white and develops very large claws. It is 

 about six inches in total length. 



The Lemming Mouse (St/naptomys) may also be found on 

 the Aleutians. It is dark gray. Its total length is a little less 

 than five inches, and the tail, which lacks long hairs, measures 

 about three-fourths of an inch. The upper incisors are grooved. 



The Meadovs^ Mouse (Microtus), often called vole, is found 

 in Japan and the Kuriles. The tail is longer than those of its 

 relatives described above, but measures less than two inches ; the 

 total length of the animal is about five inches. 



The Red-backed Mouse (Clethrionomys) , found in the Ku- 

 riles and Japan, is about the size of the meadow mouse, but is 

 usually characterized by a rusty band down the back, contrast- 

 ing with the grayish sides. Its teeth are slightly different from 

 those of the meadow mouse. As in the rest of the group de- 

 scribed above the cheek-teeth have a strongly marked zig-zag 

 pattern, but they are rooted. 



On the Galapagos Islands, off Ecuador, there are several 

 species of rats belonging to the genus Oryzomys, abundant in 

 Central and South America, and closely related to our rice rats. 

 They must have reached those islands by drifting on natural 

 rafts, for they were there before men came to the Galapagos 

 and had developed into distinct species. Since the distance in- 

 volved is more than five hundred miles, their presence on these 



