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Show 1892.] CEREBRAL HEMISPHERES OF RODENTS. 607 studied ; but it is quite impossible to characterize the group by the absence of this fissure; for instance, there is a distinct trace of this fissure in Coeloyenys and in Capromys; Dasyprocta has quite a respectable Sylvian fissure, though, as I have already pointed out, it is not thoroughly certain which of the two fissures to be found in this neighbourhood is to be compared with the Sylvian fissure of other Mammals ; but the best exception to M . Dareste's unfortunate generalization is shown by the genus Lagostomus, where the Sylvian fissure is so deep and extensive that it could not possibly be overlooked, and moreover appears on the dorsal aspect of the brain, as shown in the drawing which I have already exhibited (fig. 2, p. 599). I do not think it worth while to follow M. Dareste into his comparison of the Rodent brain with that of the Marsupials and more particularly of the Kangaroo; he chiefly bases this comparison upon the supposed absence of the Sylvian fissure in the latter animal, a supposition which is not true (see for example the woodcut (fig. ii.) illustrating Sir William Turner's paper upon the mammalian brain). § 2. Comparison of the Brains of the Genera described inter se. The convolutions of the Rodent's brain can be satisfactorily compared; but unfortunately most genera have small brains which either show no traces of any furrows or only traces. The furrows are, however, well developed in Ccelogengs, Dasgprocta, Dolichotis, Lagostomus, and Hystrix; traces of the more important furrows can be recognized in Castor, Aulacodus, Capromys, Sphingurus, Myopotamus, Lepus, Cavia, and Ociodon; the brains of Sciurus, Dipus, Gerbillus, and Cricetus are quite smooth. The brain of Dasyprocta is a convenient starting-point; I shall therefore briefly recur to the furrows which mark the cerebral hemispheres of this Rodent. I do not propose to call these furrows by any names, for that would imply a direct comparison with the similarly named furrows in the brains of other Mammalia ; I doubt very much how far such comparisons can be safely made. Each hemisphere has five furrows (apart from the " rhinal" furrow, which I leave aside for the present), three of which run parallel with the long axis of the brain and two somewhat transversely to that axis. The most important of these furrows (a, see figs. 2, 3, & 4, pp. 599, 602, 604) runs at a distance of about 5 mm. from the interhemispheral sulcus nearly from end to end of the brain. The second longitudinal furrow (b) is very short, about 5 mm. long, and about 5 m m . distance from a. The third longitudinal furrow (c) is about twice the length of the last, and is situated about midway between the dorsal and ventral surfaces of the brain when the brain is seen in profile. At the end of the first third of the hemisphere the furrow a gives off a transverse furrow (d), which joins the rhinal fissure; behind this, at the end of the second third, is a second transverse fissure which joins a on one |