140 GLIRES. 
Hab. Paumarctic Recion, North America, from Alaska and Canada southwards?.— 
Mexico (Berlandier?), Sierra Madre (U.S. Bound. Surv.’), Matamoras (Berlandier, 
U.S. Nat. Mus.’). 
The naturalists of the United-States Survey met with the Beaver on the Mexican 
boundary, it being especially plentiful in the valley of the Gila river; the most ele- 
vated point at which they found its traces was in the Colorado Cafon, in the Sierra 
Madre, about 5000 feet above the sea-level’. Mr. Allen observes that he has not been 
able to ascertain its exact limit south of the Rio Grande, but that it is well ascertained 
that its range extends for some distance into Mexico; and he adds that the manuscripts 
of the late Dr. Berlandier attest its existence in various parts of that Republic?. The 
only preserved Mexican specimens of which I have been able to learn are two skulls 
obtained by the last-named naturalist at Matamoras, and now in the United-States 
national collection at Washington’”?. 
Fam. III. MURIDE. 
1. MUS. 
Mus, Linneus, Syst. Nat. i. p. 79 (1766), as restricted by Waterhouse, Zool. Voy. ‘ Beagle,’ i. 
p. 75. 
As already remarked, the Muride of Central America are almost exclusively Nearctic 
in their affinities. The first genus however, that of restricted Mus, throws no light on 
geographical distribution ; for none of its members are indigenous to the New World, 
although four house-haunting species have been unwittingly introduced from Europe. 
The true Rats and Mice differ from all the native American forms in the structure of 
their molars, in which each transverse ridge, in an unworn state, is composed of three. 
tubercles, instead of showing two only, as in Hesperomys and its allies. The species 
established in our subregion may be recognized by the following characters :— 
1. M. rattus. Tail longer than the head and body, ears half as long as the head; 
upper parts greyish black, lower parts hardly paler. Length of head and 
body about 7”, of tail 7'-50. 
2. M. alexandrinus. ‘Tail and ears as in last species ; upper parts dark grey, lower 
parts and feet yellowish white. Head and body about 7", tail 8”. 
3. M. decumanus. ‘Tail shorter than head and body, ears one third the length of 
the head; upper parts greyish brown, lower parts greyish white, feet dusky. 
Head and body 9", tail 7'50. 
4. M. musculus. Tail about as long as head and body, ears half the length of the 
head ; upper parts brownish grey, lower parts slightly paler. Head and body 
about 350, tail 3!°50. . 
