474 RODENTIA 
Ears and eyes large; muzzle naked at the extremity. Fur soft, in 
some cases intermingled with spines. Pollex with a short nail in 
place of a claw. No cheek-pouches. Tail long, nearly naked, 
with rings of overlapping scales. Vertebre: C7, D13,L6,S 4, 
C 26-32. 
This genus is the largest in the whole mammalian class, com- 
prising not less than 130 species, ranging over the whole of 
the Old World, with the noteworthy exception of Madagascar. 
On the whole, the species are more numerous in tropical than 
in temperate regions, and very few occur in cold countries. 
Many of the species living in warm climates have flattened spines 
mingled with the fur; these spines being shed in winter, when a 
warmer covering is necessary, and replaced by hair. Five species 
occur in England, which are briefly noticed below; and it may be 
observed that none of the species are much larger than Jf. decumanus 
or smaller than J. minutus. As a rule the habits of the species 
are similar to those of the English forms, but a few are arboreal, 
while others again, like the one represented in Fig. 211, are 
aquatic. The earliest known representatives of the genus (exclud- 
ing -Leanthomys gaudryi of the Lower Pliocene Pikermi beds of Attica) 
occur in the Pleistocene of Europe. 
The Brown or Norway Rat (Jf decumanus) is a heavily built 
animal, growing to § or 9 
inches in length, with a 
bluff rounded head, small 
ears (Fig. 212, 4), and a 
comparatively short tail, 
which is always shorter 
than the head and body 
combined, and generally 
not longer than the body 
alone. The colour is a 
uniform grayish - brown 
above and white below, 
the ears, feet, and tail being 
flesh coloured. Black 
varieties, which are often 
mistaken for true Black 
Rats, are by no means rare, 
but the differences in size 
Fic. 212.—a, Head of Brown Rat (M. decumanus). and proportions form a 
B, Head of Black Rat (Mus rattus). ready means of distinguish- 
; ing the two. The Brown 
Rat is believed to be a native of Western China, where a race 
(Vv. humiliatus) has been discovered so like it as to be practically 
indistinguishable. Both this, and the next species agree in their 
