qt RODENTIA 
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-Rats 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 structuralimportance. 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- 
matie arch varies in its 
Fro. 194.—Skull of Hystrix cristata (juv.) t, Temporal degree of development, 
muscle ; m, masseter; m‘, portion of masseter transmitted and the position of the 
through the infraorbital foramen, the superior maxillary . baa 
nerve passing outwards between it and the maxillary bone. jugal 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 
backwards—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 (Bathyergine), it is extremely narrow 
transversely, its width being less than that of one of the molar teeth. 
Auditory bulle are always present, and generally large ; in some 
genera, as in the Gerbilles and Jerboas, there are also supplemental 
