XV SKULL OF RODENTS 4601 
encircling ridge is broken up into tubercles, which gives to the 
tooth a striking likeness to those of Ornithorhynchus, Other 
genera have teeth lke those of many Ungulates. It has been 
shown by Sir J. Tomes! that the minute structure of the enamel 
differs in different groups of Rodents. 
The skull shows certain primitive characters. In the first 
place there is no distinction between the orbital and the temporal 
fossa.” The sutures between the bones retain their distinctness 
for very long. Other characteristic features are the following : 
The nasals are large, and so are the paroccipital processes. The 
palate in front of the molars is not distinct from the sides of the 
skull, its edge gradually becoming rounded off above. It is also 
very narrow. The premaxillae are large in relation to the great 
incisors. There is often a very much enlarged infra-orbital foramen 
through which passes a part of the masseter muscle. The jugal 
bone lies in the middle of the zygomatic arch, which is complete 
and enormously enlarged in the Spotted Cavy (Coelogenys paca). 
As in many Marsupials, the jugal bone sometimes extends back- 
wards to the glenoid cavity, where the lower jaw articulates. It is 
usually said with an absolute want of accuracy that the cerebral 
hemispheres of the Rodents are smooth and without convolutions. 
This error has been repeated again and again in text-books. As 
a matter of fact the cerebral hemispheres of many forms are quite 
well convoluted,’ the degree of furrowing corresponding, as in so 
many groups of mammals, with the size of the animal. This at 
any rate is generally true, though the large Beaver with its scant 
convolutions is an exception. The smaller forms, such as Jus, 
Sciurus, Dipus, and Cricetus are quite smooth-brained. The best 
furrowed brain of any Rodent which has been examined is that of 
the huge Hydrochoerus. The Sylvian fissure is very generally 
not pronounced; but is particularly well-marked in Lagostomus. 
In all, or in most, Rodents the hemispheres are separated by an 
interval from the cerebellum, the optic lobes being visible between 
the two. 
The mouth cavity of this group of mammals is divided into 
two chambers by a hairy ingrowth behind the incisors; this 
arrangement is useful for animals which use their strong incisors 
1 Phil. Trans. 1850, pt. ii. p. 529. * Seen, however, in Chaetomys. 
3 See Beddard, Proc. Zool. Soc. 1892, p. 596, and Gervais, Jowrn. Zool. i. 1872, 
p- 450. 
