OBSERVATIONS ON THE RODENTIA. 93 



the apex of which is produced and forms an acute angle, 

 which almost invariably terminates beyond the condyle. The 

 rami of the jaw somewhat suddenly diverge behind their junc- 

 tion at the symphysis menti, which is of considerable extent. 

 The horizontal ramus is separated from the alveolar portion 

 beneath, by a groove, which is more or less distinct. The 

 coronoid process is usually small or but slightly elevated 

 above the level of the molars, and situated more forward than 

 in section I., and the condyloid is comparatively short. 



The principal genera, are Bathyergiis, Poephagomys, Octodon, Abrocoma, 

 Myopotamus, Capromys, Echimys, Aulacodus, Hystrix, Dasyprocta, Chin- 

 chilla, Cavia and Hydrocharus. 



The greater portion of the genera here mentioned possess 

 all the characters combined which have just been pointed out. 

 Some, however, will not agree with the description in all 

 respects, but it is a curious fact that in these instances the 

 remainder of the distinguishing characters are more than usu- 

 ally evident. In Bathyergus for instance, the posterior por- 

 tion of the descending ramus is rounded, and not acute ; but 

 here, this portion of the jaw is thrown out from the alveoli of 

 the great inferior incisors in an extraordinary degree, and 

 there is a very distinct broad channel on the under side of the 

 jaw separating the two portions in question — a character 

 never found in the species of the first section. The coronoid 

 process is also very small. Again, in the cavies {Cavia) the 

 lower boundary of the descending ramus is not thrown out 

 from the outer side of the alveolar portion ; the angle of the 

 jaw however, is greatly produced, the condyloid process is 

 short and the coronoid is very small. It is not, therefore, by 

 any one particular character that I would pretend to define 

 the sections, but by the combination of characters. 



Section III. — Leporina. The hares [Lepus) are remarkable 

 for the flatness of the rami of the lower jaw, and their great 

 size compared with the teeth, the almost horizontal direction 

 of the symphysis menti, the great height of the condyloid por- 

 tion — along the outer side of which there is an elevated ridge 

 which represents the coronoid. The condyloid process is also 

 more upright than in other rodents. The descending ramus 

 is very large and flat, and has the lower margin rounded or 

 angular, as in the genus Lagomys, at least in some of the 

 species. 



