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 Coelogenys aud in Gapromys', Dasyprocta has quite 

 a respectable Sylvian fissure, though, as I have already pointed out, 

 it is not thoroughly certain vfhich of the two fi.ssures 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 Coelogenys, Dasyprocta, Dolichotis, 

 Lagostomus, and Hystrix ; traces of the more important furrows 

 can be recognized in Castor, Aulncodus, Capromys, Sphingiirus, 

 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 (6) is very short, about 5 

 mm. long, and about 5 mm. distance from a. The third longi- 

 tudinal 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 



41* 



