The Mammals of Colorado 
comprising over 170 species and is confined to the Old World, 
but the semi-domestic House Rats and Mice have been intro- 
duced almost everywhere into the New by man's agency. 
Key of the Species 
A. Larger, total length about 12.0; tail a little shorter than the 
head and body. M. norvegicus, p. 138 
B. Smaller, total length about 7.0; tail longer than the body. 
M. musculus, p. 139 
Mus norvegicus (Lat. from or of Norway). Brown Rat 
Mus norvegicus Erxl., Syst. Regn. Anim., i., p. 381 (1777). 
Type locality. — Norway. 
Measurements. — Total length, 12.0; tail vert., 5.6; hind foot, 1.5. 
Description. — Above rusty grayish brown; sides grayer; beneath, 
ashy white ; tail above dusky, beneath paler. 
The Brown Rat can be easily distinguished by its size, the short- 
ness of its ears and tail, and by its coarse fur. 
Distribution. — It is now practically cosmopolitan, having spread 
inland in various countries from the seaports to which it has been 
brought from Europe by ships. In Colorado it is found in the 
larger towns, such as Denver, Colorado Springs, Pueblo, Boulder, 
Longmont, and Greeley, but so far is not known to have spread 
into the country. 
Habits. — The Brown Rat is well known as a pest about 
houses and other buildings, but thus far has not gained a 
very strong foothold in Colorado. Its omni\'orous appetite, 
and its great adaptability to circumstances enable it to live 
anywhere. It is a pest about storehouses, for it destroys 
much, and also about granaries it does much damage. It 
has been in the past few years discovered to be a means 
of spreading the bubonic plague, and in consequence of that 
discovery strong efforts are being made in San Francisco 
and others of our Pacific Coast ports to destroy the rats 
which are always very numerous in seaport towns. A similar 
crusade has also been waged against them in the Philippines, 
in India, and at Cape Town, South Africa. 
