MAMMALIA. 465 



race, as well as the black rat, M. vattus {pi. 113, fig. 7), which lias the 

 same origin. The latter is now replaced by the Norway or brown rat {M. 

 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 manner as 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, Af. sylcaticus {pi. 113, fig. 6), is never found to reside in the 

 houses. 



The genus Gerhillus (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 Criadus (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 Europe. PL 113, fig. 

 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 Eiver 

 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 (iY 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 Eocky 

 Mountains. 



A third species has recently been detected in Eockland 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. 



66& 



