610 IVIR. F. E, BEDDARD ON THE [DeC. 6, 



d. Dasyprocfidse. 



Dasyprocta. 

 Ccelogenys. 



e. Caviidae. 



Cavia. 



Dolichotis. 



Bydrochcerus. 



It will be noticed at once that the convoluted brains belong- to the 

 last of the three divisions of the Simplicidentati ; the obscure dints 

 upon the upper surface of the cerebrum of the Beaver are apparently 

 the only apology for convolutions possessed by either the Sciuro- 

 morpha or the Myomorpha. In point of size there is every reason 

 why some of the members of these two groups should have 

 recognizable convolutions ; apart from the Beaver, which is one of 

 the largest of the Rodentia, there is the Common Squirrel, whose 

 brain is distinctly bigger than that of Octodon, in which genus there 

 are decided traces of convolutions ; it seems to me therefore that — 

 for the present at least — we may regard these two groups of the 

 Simplicidentati as being characterized by the entire absence of 

 convolutions. It is quite otherwise with the Hystricomorpha ; the 

 larger members of this group show convolutions which can be 

 reduced to a common plan. But there are differences among the 

 genera which permit of an arrangement in accordance with the 

 varying condition of the convolutions. The Hystricidse form a 

 perfectly natural familj'. Their brain is characterized by its peculiar 

 shape, rounded in front, and by the fact that the convolutions for 

 the most part are transverse and not longitudinal in direction. Only 

 in Hyittrix itself is the principal longitudinal furrow a, which occurs 

 in all other Rodents whose brains show any convolutions at all, 

 represented to any extent. In relation to this fact it may be 

 pointed out that we may fairly regard Hystrix as the least special- 

 ized of the Rodents whose brains are treated of here. The only 

 animal whose brain approaches that of the Porcupines is, as I have 

 already pointed out, Capromys ; the brain of this Rodent is rounded 

 off in the same way anteriorly. The convolutions are so feebly 

 developed that the comparison can perhaps hardly be pushed any 

 further. It may, however, be worth remarking that a lateral furrow, 

 which I regard as being a part of the Sylvian fissure, is well marked 

 both in Capromys and in the Porcupine. This latter furrow is also not 

 inconspicuous in Aulacodus (fig. 6, p. 611), the relationships of which 

 to Capromys are apparent from a comparison of their brains. Even in 

 J/2/o;?o^a?«Ms, where the shape of the brain is very different from that of 

 other Octodontidae, there is a distinct trace of the lateral oblique furrow 

 which is so well marked in both Aulacodus and Capromys. In any 

 case, therefore, the naturalism of the family Octodontidae is shown by 

 a comparison of their brains. We next come to the Dasyproctidae, 

 represented by the tAvo genera Djsyprocta and Ccelogenys. Of these 

 two brains Ccelogenys is the larger, and yet it has the fewest furrows ; 



