1892.] CEREBRAL HEMISPHERES OF RODENTS. 601 



hemisphere I could only recognize one of these. On the right half 

 of the brain there is a marked indentation to the outside of the 

 posterior extremity of the longitudinal furrow. 



On the under surface of the brain the rhinal fissure is seen to be 

 Tery distinct and complete. The Sylvian fissure is not so distinct 

 where it joins the rhinal fissure as it is laterally ; just in front of 

 the Sylvian fissure is a small fissure arising from the rhinal fissure, 

 which appears to be the termination of the anterior transverse 

 fissure of the dorsal surface of the brain. 



Sphingurus prehensilis. — Total length 36 mm., breadth 27 mm., 

 height 19 mm. 



The cerebral hemispheres show the same peculiar shape that is 

 seen in Hystrix ; their contour is almost circular ; the posterior 

 divergence of the two hemispheres is so nearly filled by the projecting 

 cerebellum that the merest peep at the corpora quadrigemina is 

 alone possible. 



Each hemisphere measures 24 mm. in length ; the upper surface 

 is smooth, but not so smooth as in the Beaver ; it is marked by a 

 good many meandering lines which are for the most part directed 

 downwards. I do not describe these particularly as they do not 

 appear to me to be of any morphological importance. In addition 

 to these there are two dents on the upper surface of the brain. 

 The posterior of these is placed at a distance of about 6 mm. from 

 the hinder margin of the brain ; the anterior mark is about the 

 same distance in front. 



The rhinal fissure is not well marked, but it is complete. The 

 Sylvian fissure was hardly apparent. 



Sphingurus villosus. — Total length 36 mm., breadth 30 mm., 

 height 20 mm. 



This brain is almost exactly identical in size with that of the last 

 species ; nor is there any occasion to describe the general sha[)e 

 and the proportions of its various regions, for they are precisely 

 as in Sphingurus prehensilis. 



On the other hand, the brain is deeply furrowed in comparison 

 with that of the other Sphingurus. It appears to me to be a very 

 remarkable fact there should be this difference in the brain-surface of 

 two animals of the same size and belonging to the same genus. The 

 rhinal fissure is quite deep and perfectly complete. The Sylvian 

 fossa is much deeper than in Sphingurus prehensilis. The Sylvian 

 fissure is also quite conspicuous and passes on to the dorsal side of 

 the brain, where it is deeper than at its origin ; this region of the 

 Sylvian fissure is represented by the posterior of the two dents upon 

 the surface of the brain of Sphingurus prehensilis. In front of this 

 fissure are two others equidistant from it and from each other, 

 which run in a similar direction ; besides these principal fissures 

 are numerous smaller ones which are principally branches of them. 

 There was no longitudinal fissure. 



