1892.] CEREBRAL HEMISPHERES OP 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. 



Coelogenys (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 (Z as a deepish 

 depression connected by a shallow furrow with the end of the first 

 part of a. 



In Hystrix 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 obliqjie 

 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 my 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 classifi- 

 cation propounded by the late Mr. Alston. This naturalist divided, 

 it will be recollected, the Order Rodentia into three groups: — 



(I) HeBEDENTATI ; (2) SiMPLICIDENTATI ; (3) DuPLICIDENTATI. 



We 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 exam- 

 ined brains, with the genera which I have examined: — 



(1) SCIUROMORPHA. 



Sciurus. 

 Castor. 



(2) Myomorpha. 



Gerbillus. 

 Cricetus. 

 Dipus. 



(3) Hystricomorpha. 

 a. Octodontidse. 



Octodon. 

 Myopotamus. 

 Capromys. 

 Aulacodus. 



b. Hystricidse. 



Hystrix. 

 Sphingurus. 



c. Chinchillidse. 



Lagostomus. 

 Chinchilla. 



