RODENTIA ^^^ 



even greater freedom of movement of the articulation of the jaw 

 than in the rabbit, and in particular the attachments of the 

 muscles are shifted so that they can pull the jaw forward. 



There is a great deal of adaptive radiation within the order, 

 which includes land-living and burrowing forms, aquatic animals 

 such as the beaver {Castor fiber), saltatorial kangaroo-like 

 creatures such as the jerboas, the semivolant flying squirrels 

 with a long patagium stretched from limb to hmb, and the 

 aberrant spiny porcupines. In Britain we have only some of 

 the less remarkable species, chiefly the rats and mice. The genera 

 Mus (house mouse), Rattus (black and brown rats), Apodemns 

 (field mice) and Micromys (harvest mouse) make the sub-family 



Murinae, with dental formula and cheek teeth with closed 



1003 



roots and a crown pattern of three transverse ridges. Rattus 



and Mus are both human importations to this country, and to 



some extent at least dependent on man for their continued 



existence. The house mouse (M. musculus) became extinct on 



St. Kilda a few years after the human population left. The black 



rat {R. rattus) was formerly important because its fleas, Xenopsylla 



cheopis, carried the germs of plague, as they still do in the East. 



The Microtinae or voles have the same number of teeth as the 



mice, but the roots of the cheek teeth remain open permanently 



or until late in life. The crowns are worn flat by the food, which 



is largely grass, and show a characteristic pattern of transversely 



elongated triangles. The short-tailed or field vole [Microtus 



agrestis) is probably our most numerous mammal, but as it 



spends most of its time in runs which it makes in long grass it is 



seldom seen. 



The dormouse {Muscardinus avellanarius) has dental formula 



T /-V T- O 



— —, and is notorious for the depth and length of its hibernation, 

 1013 



during which it consumes much of its stored fat. Its European 

 relative Glis glis, which was fattened for the table by the Romans 

 on chestnuts and walnuts and served with honey sauce, has been 

 introduced into England and is numerous in parts of Hertford- 

 shire and Buckinghamshire. The dormouse is arboreal, and, as its 

 specific name correctly implies, it inhabits chiefly the hazel trees, 

 which make a second layer in so many oakwoods. More character- 

 istically confined to the trees are the squirrels, of which the only 

 native British species, the red squirrel (Schirus vulgaris) seems to 



