xv SKULL OF RODENTS 461 



encircling ridge is broken up into tubercles, which gives to the 

 tooth a striking likeness to those of Ornithorliynchus. Other 

 genera have teeth like those of many Ungulates. It has been 

 shown by Sir J. Tomes 1 that the minute structure of the enamel 

 differs in different groups of Eodents. 



The skull shows certain primitive characters. In the first 

 place there is no distinction between the orbital and the temporal 

 fossa. 2 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 (Codogenys 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 Eodents 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, 3 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 Mus, 

 Sciurus, Dipus, and Cricetus are quite smooth -brained. The best 

 furrowed brain of any Eodent 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, Eodents 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. 2 Seen, however, in Chaetomys. 



3 See Beddard, Proc. Zool. Soc. 1892, p. 596, and Gervais, Journ. Zool. i. 1872, 

 p. 450. 



