VOLES. 265 



tlie larger of the two. It was more than twice the size of the Bearer (the length of its skull, for 

 example, was nine inches, while that of the Beaver is only four). The Trogontheriusi, again, was 

 only about a fifth larger than the Beaver. 



Arvicolin-s. (Map 84.) 



The lower-jaw bones of two or three species of fossil Aryicol^e have been found in the bone 

 breccia of Goslar. 



Musk Rat. (Fiber Zibethicus.) (Map 86.) Although unquestionably one of the Arvicolix-B, 

 there is a good deal in the Musk Rat which reminds us of the Beaver. Its fur is similar, it passes 

 the most of its time in the water ; its tail also is naked and scaly, only narrow instead of broad, and 

 placed vertically instead of horizontally. It inhabits the whole of North America, from the Atlantic 

 to the Pacific and from the Rio Grande to the Barren Grounds of Arctic America. It is abundant in 

 Washington territory, and to the north of that district on to Bhering's Straits, and in the north 

 of the Rocky ^Mountains, but has not yet been met with in California. Dr. Newberrj- says : " In 

 the Sacramento vallej', in the Klamath Lake region, in the basin of the Des Chutes, — j)lace8 

 apparentl}- fitted by nature to be jjaradises of Musk-Rats ; shallow, rush-grown lakes, and rush- 

 bordered, canal-Uke streams, just where, in the Eastern states, Musk-rats woidd abound, — though 

 I looked carefully I never saw the animal, his track, his habitations, nor even his characteristic 

 heaps of emptied shells of Zhtio and Aimdoiita. I therefore concluded that in all this region the 

 Musk-Rat does not exist."* Mr. Lordf describes a second species. Fiber Osoyoosensis, as found in 

 the Rocky Moimtains and at Cascade Mountains, but the distinctions taken by him scarcely seem 

 specific. 



It was at one time sujiposcd that tlie Musk-Rat was found on the Asiatic side of Bhering's 

 Straits, but it appears now to be ascertained that the skins obtained from the Tschucktchis of Kamts- 

 chatka are procured from the tribes on the American side of tho Straits. 



Voles. — (Map 84.) The Field-mice, or Toles, are iiumerous in species, which are spread over 

 the northern hemisphere through America, Europe, and Asia, and in number of individuals they 

 jjrobably far exceed an}- other mammal. Inhabiting very nearly the . same territories as the true 

 Mice and Rats, they each have a tendency in an opposite direction. The true Mice rather affect 

 the warmer parts of the temperate zone, the Field-mice prefer the colder ; as in plants we see 

 species occupying successive bands of latitude — the spruce-fir, for example, stretching in a broad 

 band across the north of Europe, and the silver-fir doing the same in the middle of Europe, 

 so these two families repeat something of the same sort in the animal kingdom, but so mixed to- 

 gether that it is not easy to prove it otherwise than by pointing to tho fact that the extreme 

 northern and cold districts have Field-mice, and no true Mice ; and the southern and warmer 

 districts have true Mice and no Field-mice. Thus we have no true Mice in Greenland and the 

 eirciimpolar region, but we have the Lemmings, a genus of Field-mouse. So at the equator, 

 and in the tropics, we have true Mice, but no Field-mice. On the cold and lofty stepj^es of 

 Mongolia and Central Asia true Mice are absent, and their place is supplied by Field-mice. 

 Two species occur at some elevation on the southern slope of the Himmalayahs, stragglers from 

 Europe and Central Asia. In the deserts of Sahara Field-mice are unknown, but true Mice occur 



* Newberry, "Report in U.S. Pac. Railroad ExpL," vol. vi. p. 22. 

 t Lord in "Proc. Zool. See." ISG3. 



M M 



