1892.] CEREBRAL HEMISPHERES OF RODENTS. 603 



(on the right half), the Sylvian fissure also joins this longitudinally 

 running furrow. 



A second longitudinal furrow, at about the same distance from the 

 first described longitudinal furrow as the latter is from the median 

 interhemispheral furrow, and commencing about 5 mm. from the 

 posterior margin of the brain, has a course of 4 or 5 mm. ; this 

 furrow is rather more strongly marked on the right half of the brain. 

 Again, to the outside of this is a longer furrow but less strongly 

 marked, which commences at the very margin of the brain just 

 opposite to the point where the rhinal furrow is lost beneath the 

 corpora quadrigemina ; this furrow is situated about twice as far 

 from the second longitudinal furrow as that furrow is from the first. 

 Its length is about 12 mm. 



On the under surface of the brain the hippocampal gyrus is seen 

 to be very prominent in the temporal region, and when the brain is 

 viewed laterally this convex projection is very apparent. The rhinal 

 fissure is complete and anteriorly appears to give oiF a short 

 forwardly running branch such as I have described in Coelogenys. 

 The Sylvian fissure is very slightly marked where it joins the rhinal 

 fissure. 



The olfactory bulbs are large. 



Leuret and Gratiolet's figure (8, pi. iii.), though in my opinion 

 better than that of Owen, is not so clear as the drawing which I 

 exhibit. 



Ccelogenys paca. — Total length 53 mm., breadth 42 mm., height 

 23 mm. 



The cerebral hemispheres diverge posteriorly so as to display 

 the corpora quadrigemina ; there is no divergence anteriorly. Each 

 hemisphere measures 34 mm. in length and is considerably wider 

 behind than in front, the diameter increases more rapidly after the 

 Sylvian fissure. 



The surface of the hemispheres (fig. 4, p. 604) is indented by 

 a few very deep furrows, which are quite symmetrically arranged. 

 A furrow 10 mm. long lies posteriorly at a distance of 7 mm. from 

 the interhemispheral sulcus ; anteriorly there is a shorter furrow 

 which suddenly bends outwards posteriorly and runs almost parallel 

 with the margin of the hemisphere. The posterior furrow is con- 

 tinued forward by a very faintly marked furrow which approaches 

 the middle line and then turns outwards, joining or running just 

 behind the end of the anterior furrow. On the right side of the 

 body there is a small but deep indentation on the inner side of the 

 posterior furrow, between it and the interhemispheral furrow ; there 

 is also (on both sides of the brain) another dint — it is hardly long 

 enough to be called a furrow — on a level with the hind extremity 

 of the posterior furrow and about as far from it as that furrow is 

 from the median interhemispheral sulcus. 



Outside this again is a shallow furrow which begins at the 

 posterior margin of the hemisphere, just on a level with the end of 



