1899.] 



BRAUr OF HYDEOCHOEKXJS. 



801 



parts, an anterior shorter, and a posterior longer region. It 

 is the fissure lettered a of my former paper — the most prevailing 

 fissure o£ the Rodent brain. 



In a second brain (fig. 2, p. 800) the fissure in question is also 

 broken on the left side by a bridging convolution, in this case on the 

 right side also, as is the case with the brain figured by Dareste. It 

 will be noticed that this furrow posteriorly approaches the median 

 furrow of the brain, that dividing the hemispheres, and is very 

 nearly lost to sight over its edge. 



In the third brain (fig. 3) the furrow a is again broken on the 

 left side, but complete upon the right. Furthermore the furrow 

 (fig. 3) completely disappears from view posteriorly, and this region 

 of the gyrus, ■which is bounded externally by the furrow in question, 

 is very distinctly depressed below the general surface of the brain. 



Kg. 3. 



Cerebral hemispheres of Hydrochxrus, dorsal view. Nat. size. 



The conditions that have just been described seem to give a clue to 

 thenatureof this sulcus and gyrus. The lobus hippocampi narrowing 

 as it passes backward turns up the back of the brain and becomes 

 continuous with this gyrus, which I therefore consider to be the 

 hippocampal gyrus appearing upon the dorsal surface of the brain. 

 In this feature the brain of the Capybara resembles that of certain 

 Carnivora ^ and of certain Ungulata ^. 



The fissure which I letter b is, as in other Eodents, short ; it is, 

 however, quite deep and well-marked. It runs obliquely inwards 

 in all three brains. 



» Helictis (Garrod, P. Z. S. 1879, p. 307) ; Gulo (Beddard, P. Z. S. 1895, 

 p. 143), &c. &c. 



* Mosehus (Flower, P. Z. S. 1875, p. 174) ; Dicotylea (lurner, Journ. \rxa,r, 

 1891, p. 134), &c. &c. 



