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Show 1892.] CEREBRAL HEMISPHERES OF RODENTS. 609 point where d would join it were that furrow present ; the Sylvian fissure is deep and bifurcate at the upper end, nearly joining a. Ccelogenys (fig. 4, p. 604) has a less convoluted brain. The fissure a is divided into two quite separate portions, which occupy the first and last thirds of the hemisphere; b is merely a deep dent which is rather transverse in direction, running towards c: neither d nor the Sylvian fissure was well developed ; on one side, however, the latter could be made out, and there were indications of d as a deepish depression connected by a shallow furrow with the end of the first part of a. In Hystriv a is not extensive; fissures which possibly correspond to b and c are present but run obliquely outwards; c and d are well developed, so that altogether the furrows have a markedly oblique disposition. In the smaller brains a is, as in the larger brains, the most important fissure upon the upper side of the brain. It is the only one present in Octodon and Myopotamus. § 3. The Structure of the Hemispheres and the Classification of the Rodentia. It is not m y object here to enter into any detailed account of the various ways in which this group of Mammals has been arranged by various authorities; I shall only point out how far the results which I have been able to get together affect the scheme of classification propounded by the late Mr. Alston. This naturalist divided, it will be recollected, the Order Rodentia into three groups: - (1) IlEBEDENTATI ; (2) SlMPLICIDENTATI ; (3) DuPLICIDENTATI. W e are here only concerned with the last of the three groups, which are separated by Alston into a number of families. The following are the families of which I have personally examined brains, with the genera which I have examined :- (1) SCIUROMORPHA. Sciurus. Castor. (2) MYOMORPHA. Gerbillus. Cricetus. Dipus. (3) HYSTRICOMORPHA. a. Octodontidae. Octodon. Myopotamus. Capromys. Aulacodus. b. Hystricidae. Hystrix. Sphingurus. c. Chinchillidae. Layostomus. Chinchilla. |
