MAMMALIA. A65 
race, as well as the black rat, I rattus (pl. 118, fig. 7), which has the 
same origin. The latter is now replaced by the Norway or brown rat (J/. 
decumanus), which did not appear in Europe until the eighteenth century, 
and has now become more abundant than the black rat. It has also 
reached America in the same manneras the two preceding species. 
Besides these three introduced species America possesses several others 
(five are well ascertained) which are peculiar to her. The field mouse of 
France, M. sylvaticus (pl. 118, jig. 6), is never found to reside in the 
houses. 
The genus Gerbillus (the gerbils) has three molars that differ very little 
from those of rats. Their superior incisors are furrowed with a groove. 
Their hind feet are somewhat longer in proportion than those of rats in 
general. The tail is long and hairy. 
In the genus Cricetus (hamsters) the teeth also differ very slightly from 
those of rats, but the tail is short and hairy, and on each side of their mouth 
internally are sacs or cheek pouches in which they transport the grains they 
-collect to their subterranean abodes, accumulating large heaps. Most of 
the species of this genus inhabit the northern parts of Hurope. Pl. 118, fiz. 
9, represents the common hamster (C. vulgaris). 
The genus Sigmodon has three molars above and below on each side, 
provided with small roots, and very profound, alternate folds towards the 
summit. The tail is hairy, the feet simple, the fore feet four-toed with 
the rudiment of a fifth; the hind feet five-toed. Two species of this 
genus exist in North America; one is S. hispidum or the cotton rat from 
Florida, very numerous in the deserted plantations lying on the River 
St. John, particularly in the gardens. Its burrows are seen in every 
direction. 
The genus Neotoma embraces two North American species. The grind- 
ing surfaces of the molar teeth differ somewhat in this genus from what 
they are in Arvicola, and their large roots constitute.a very essential char- 
acter. The fore feet are provided with four toes and the rudiment of a 
fifth ; the hind ‘ones are five-toed. The tail is hairy. 
The Florida rats (NV. floridana) in Florida burrow under stones and the 
ruins of dilapidated buildings. In Georgia and South Carolina they prefer 
remaining in the woods. In some swampy situations, in the vicinity of 
sluggish streams, amid tangled vines interspersed with leaves and long 
moss, they gather a heap of dry sticks, which they pile up into a conical 
shape, and which, with grasses, mud, and dead leaves, mixed in by the 
wind and rain, form, as they proceed, a structure impervious to rain, and 
inaccessible to the wild cat, racoon, or fox. At other. times their nest, 
composed of somewhat lighter materials, is placed in the fork of a tree. 
Another species (N. drummondii) has been discovered in the Rocky 
Mountains. 
A third species has recently been detected in Rockland county, New 
York, by John G. Bell, Esq., but not characterized. Numerous remains of 
an extinct species, the largest of the genus, are found in the bone caves of 
Pennsylvania. 
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