CEREBRAL HEMISPHERES OF RODENTS. 



599 



1892.] 



breadth until about the middle point, when the margins are nearly 

 parallel, converging slightly posteriorly. The rhinal fissure is quite 

 continuous ; the Sylvian fissure is represented by the merest trace. 

 On the upper surface of the brain there are two short furrows on 

 each side, one in front of and to the outside of the other. 



Lagostomus trichodactylus. — Total length 43 mm., breadth 

 35 mm., height 18 mm. 



The cerebral hemispheres diverge posteriorly so as to display the 

 corpora quadrigemina ; they do not diverge at all anteriorly. The 

 extreme length of each hemisphere is 31 mm. The hemispheres 



Fig. 2 



^f-\\ 



Brain of Lac/ostomus trichodactylus. 

 A, from above ; B, from the side : a, b, c, lougitudinal fissures; 



S, Sylvian fissure. See p. 607. 



are wider behind than in front ; the breadth increases very gradually 

 up to the Sylvian fissure ; at this point the brain suddenly becomes 

 wider. 



The brain (fig. 2) is marked by several fairly deep furrows and 

 by other slighter ones. These are perfectly symmetrical. On 

 either side of the intercerebral fissure is a furrow 9 mm. long which 



