| OCR Text |
Show 610 MR. F. E. BEDDARD ON THE [Dec. 6, d. Dasyproctidee. Dasyprocta. Ccelogenys. e. Caviidae. Gavia. Dolichotis. Hydrochcerus. 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 Hystricidae form a perfectly natural family. Their brain is characterized by its peculiar shape, rounded in front, and by tbe fact that the convolutions for the most part are transverse and not longitudinal in direction. Only in Hystrix 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 specialized 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 Myopotamus, 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. W e next come to the Dasyproctidee, represented by the two genera Dasyprocta and Ccelogenys. Of these two brains Ccelogenys is the larger, and yet it has the fewest furrows |
