346 Mr. G. R. Waterhouse on the Rodentia. 



the lower jaw of each of the principal genera of the Hystricine 

 section, together with a view of the lower jaw of the rat, 

 which will serve for comparison. The lower jaw in the Hy- 

 stricina is remarkable, generally, for the small size and ad- 

 vanced position of the coronoid process. The condyloid is in 

 almost all the species much compressed, and of considerable 

 antero-posterior extent ; the articular surface, which is very 

 narrow, is chiefly confined to the anterior part of the condyloid 

 process — such is the case in the Dasyprocta Echimys, Octo- 

 dontidce, Chinchillidce and Caviida, but in the Hystricidce the 

 articular surface is broader. The most important character in 

 the present group, however, is observable in the angular portion 

 or descending ramus (PI. VIII. b, fig. 4.). In form, the descend- 

 ing ramus, or that portion which lies behind and below the 

 bony covering of the great inferior incisor, almost invariably 

 approaches to a triangular figure ; the posterior portion is 

 more or less deeply emarginated, and the lower posterior angle 

 is produced. The modifications in the form of this part may 

 be best described by selecting the lower jaw of Echimys (fig. 70 

 as a type, and pointing out the deviations from that type. 

 Here the descending ramus is deeply emarginated behind, the 

 emarginated portion being in the form of a semicircle : the 

 lower boundary of the ramus, when viewed from beneath, pre- 

 sents a flat surface, and is produced about the middle on the 

 inner side into an obtuse angle, as seen in the view of the un- 

 der side of the jaw, fig. *], and at «, fig. 6. The lower jaw in 

 Nelomys, Cer corny s (f. 10.), Poephagomys, Ctenomys (f. 11.), 

 Schizodon, Octodon (f. 12.), and Capromys (f. 6.) is essentially 

 the same, differing almost only in the depth of the posterior 

 emargination, in having the posterior angle a little more or less 

 produced, &c, which differences are seen by an inspection of 

 the figures. The lower jaw of Myopotamus (f. 5.) is also essen- 

 tially the same, but the descending ramus is thrown more 

 boldly outwards, its depth is less, and the width of the hori- 

 zontal plate which forms the lower boundary is greater — the 

 angle *, as at a, fig. 6, is still distinct. In the smaller size of 

 the coronoid process we observe an approach to the jaws, fig. 9 

 and 13. {Lagostomus and Cavia), which is accompanied with 

 other characters, such as the extremely contracted condition 

 of the palate between the anterior molars, exhibited in the 

 skulls of the three genera. In Lagostomus (fig. 9.), in addition 

 to a very small coronoid process, are other peculiarities, in 

 which we find a most perfect transition between the jaw of 



* This angle is found in no Rodents excepting those belonging to the 

 Hystricine section. 



