RODENT/A 



445 



mastoid bullae forming great hemispherical bony swellings at the back 

 of the skull (see Fig. 7. Per) ; and in those 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 -f-, c $, p f, m § (total 28) 

 in the Duplicidentata to i \, c #, p #, m f (total 12) in Hydromys, 

 Xeromys, and one species of Heterocephcdus ; but in the great 

 majority of forms it is very constant, i \, c %, p \<j~i\> m f 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 

 will 

 curve 

 until 

 circle 

 been 



tooth 



gradually 



upon itself 



a complete 



or more has 



formed, the 



Fig. 195. — Vertical and longitudinal section through skull of 

 the Beaver (Castor fiber) showing the cerebral cavity, the greatly 

 developed turbinal lamella;, the mode of implantation of the large 

 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 (Dasyprodidce), the milk-teeth are long retained, 

 while in the allied Cavies (Caviidce) they are shed before birth. 



There are generally nineteen dorso-lumbar vertebrae (thirteen 

 dorsal and six lumbar), their form varying in the different genera. 



