Mr. G. It. Waterhouse on the ttodentia. 



347 



Echimys and the other genera above noticed, and the Caviidce : 

 this is not only evinced in the modification in the form of 

 the descending ramus, but in the ridge exhibited in the side- 

 view of the jaw, fig. 9, which runs obliquely downwards and 

 forwards from the condyle. In the cavy (fig. 13.) the same 

 kind of ridge is observable, but it is extended more forwards, 

 and forms the outer boundary of the remarkable protube- 

 rance (a) on the outer side of the horizontal ramus, which 

 forms so peculiar a character in the Caviidce*. The upper 

 surface of this protuberance is concave. 



The lower jaw of Lagos tomus does not possess the hori- 

 zontally flattened plate which forms the inferior boundary of 

 the descending ramus in Echimys, &c, and the angle exhi- 

 bited at a, fig. 6, is here nearly lost, nor do we observe this 

 angle in the Cavies, Dasyproctas, or Hystrices. This last-men- 

 tioned group is so admirably illustrated by Brandt, that it 

 will be unnecessary here to dwell upon it. 



In the present section of Rodents, the angular portion of 

 the lower jaw (PL VIII. b, fig. 4.) is not, as in the rats (fig. 8.), 

 in the same vertical plane as the dental portion (a, fig. 4.), but 

 is attached as it were to the outer side of that part. In the 

 great section Murina, the angular portion of the jaw descends 

 from the under side of the alveolus of the incisor, whilst in the 

 Hystricine section it is at its commencement thrown outwards 

 from the lateral part of thebony covering of that tooth (PI. VIII. 

 «, fig. 4.), and when the jaw is viewed from beneath, a longitu- 

 dinal groove generally separates the dental from the angular 

 portion. This character is most strongly marked in Hystrix, 

 Dasyprocta, Capromys, Echimys, and the Octodontidce ; in the 

 Chinchillidce, and especially in the Caviidce, it is less strongly 

 marked, owing to the comparative shortness of the lower 

 incisors. 



DESCRIPTION OF PLATE VIII. 



Fig. 1 . Lower jaw of Hystrix. 



A th crura. 



Erethizon. 



Dasyprocta. 



Myopotamus. 



Capromys. 



Echimys. 



Fig. 8. Lower jaw of Mus. 



9. Lagostomus. 



10. Cercomys. 



11. Ctenomys. 



12. i Octodon. 



13. Cavia. 



[To be continued.] 



* In the Caviidce I include the genera Cavia, Kerodon, Dolickotis, Desm. 

 {ox Mara of Lesson), and Hydrochcerus. It is one of the most distinctly marked 

 groups in the Rodentia, and should not be confounded with the Dasyproctas, 

 which differ much in skull and dentition. 



