RODENTIA 445 
mastoid bull forming great hemispherical bony swellings at the back 
of the skull (see Fig. 7, Per); and in these genera, and in the true 
Hares, the meatus auditorius is tubular and directed upwards and 
backwards. The mandible is characterised by the abruptly nar- 
rowed and rounded symphysial part supporting the large incisors, 
as well as by the small size of the coronoid process and the great 
development of the angular portion. 
The dental formula varies from i 3, ¢ $, p 3, m 2 (total 28) 
in the Duplicidentata to i 4, ¢ 2, p 2, m 2 (total 12) in Hydromys, 
eromys, and one species of Heterocephalus; but in’ the great 
majority of forms it is very constant, i4,c 9, p ay m & being 
very typical. Only in the Duplicidentata is there a second pair of 
upper incisors, which are of very small size, and situated immedi- 
ately behind the large normal pair. This group is also peculiar in 
that the enamel of the incisors is not confined to their anterior 
surfaces, but extends partially on to their sides. It is by reason 
of the thick layer of enamel on their anterior surface and its 
absence from the posterior surface that the incisors maintain their 
sharp chisel-like edge, which is so essentially characteristic of the 
order. Both the upper and the lower incisors are regularly curved 
—the curvature being somewhat greater in the upper ones—and 
since they grew continuously from persistent pulps, it is quite 
evident that should any accident, such as the loss of one of them, 
or displacement by 
fracture of the jaw, 
prevent the regula- 
tion of the length 
by attrition against 
one another, the 
unopposed tooth 
will gradually 
curve upon itself 
until a complete Fic. 195.—Vertical and longitudinal section, through skull of 
circle or more has the Beaver (Castor fiber) showing the cerebral cavity, the greatly 
developed turbinal lamellz, the mode of implantation of the large 
been formed, the incisor, and the curved rootless molars. 
tooth, perhaps, 
passing during its growth through some part of the animal’s head. 
The molars, as already mentioned, may be rooted or rootless, tuber- 
culated or laminated; this diversity of structure occurring even 
in the same family. When there are more than three cheek- 
teeth those in front of the last three have succeeded milk-teeth, 
and must therefore be considered premolars. In some species, as 
in the Agoutis (Dasyproctide), the milk-teeth are long retained, 
while in the allied Cavies (Caviide) they are shed before birth. 
There are generally nineteen dorso-lumbar vertebre (thirteen 
dorsal and six lumbar), their form varying in the different genera. 
