44+ 



RODE NT! A 



quarters at the present day ; while in Australia and Madagascar 

 they are represented only by a few genera. All the Rodents are 

 exclusively herbivorous, and the whole of them gather their food 

 by gnawing. They present considerable diversity of habits. Thus 

 the Squirrels are arboreal, and some of them provided with a para- 

 chute for taking flying leaps from tree to tree ; the Hares are 

 cursorial; the Jerboas agile jumpers; the Mole -Eats fossorial ; 

 while the Beavers and Water -Voles are aquatic. In spite, how- 

 ever, of this diversity of habits the Rodents present a remarkable 

 similarity in general structure ; so much so, indeed, that the char- 

 acters employed for distinguishing the various families and genera 

 are comparatively trivial, and of slight structural importance. The 

 skull of the Rodents is characterised by the invariable presence of 

 the zygomatic arch, of which the middle portion is formed by the 

 jugal (Fig. 7, p. 37) ; and, as already mentioned, the orbit communi- 

 cates freely with the temporal fossa. There is invariably a long 

 diastema separating the incisors from the cheek-teeth ; and, with 

 the exception of the Duplicidentata, the glenoid cavity of the squa- 

 mosal is elongated antero-posteriorly. Postorbital processes of the 

 frontals exist only in the Squirrels, Marmots, and Hares ; in all 

 other genera they are rudimentary or altogether absent ; the 

 zygoma never sends upwards a corresponding process ; the lachry- 

 mal foramen is always 

 within the orbital margin ; 

 in many species the infra- 

 orbital foramen is very 

 large (in some as large as 

 the orbit), and transmits 

 part of the great masseter 

 muscle (Fig. 194, m), by 

 means of which the jaws 

 are worked. The zygo- 

 matic arch varies in its 



Fir:. 194.— Skull of Hystrix cristate, (juv.) t, Temporal degree of development, 



muscle; wi, masseter; to', portion of masseter transmitted qjhJ ^he position of the 



through the infraorbital foramen, the superior maxillary . ■ ,, ". . , 



nerve passing outwards between it and the maxillary bone. J 11 ?'^ therein IS Used as a 



distinguishing character 

 for grouping the families ; the nasals are, with few exceptions, large, 

 and extend far forwards ; the parietals are moderate, and there is 

 generally a distinct interparietal. The palate is narrow from before 

 1 lackwards — this being especially pronounced in the Hares, where it 

 is reduced to a mere bridge between the premolars ; while in other 

 cases, as in the Mole-Rats (Bathyergince), it is extremely narrow 

 transversely, its width being less than that of one of the molar teeth. 

 Auditory bulla? are always present, and generally large ; in some 

 genera, as in the Gerbilles and Jerboas, there are also supplemental 



