PROSENCEPHALON OF MAMMALS. 



105 



Didelphys Virginiana. 



has the anterior apex of the hemisphere marked off by a deeper 

 transverse fissure, extending to the inner surface. In the Her- 

 bivorous Marsupials the fissures 

 are more definite, deeper, and 

 rather more numerous in the 

 larger (Macropus major, fig. 74) 

 than in the smaller species 

 ( I lypsiprymnus). All Marsupials 

 have the hippocampal fissure, 

 fig. 46, 4, fig. 73, i, coextensive 

 with the antero-posterior range of the prosencephalic cavity, and 

 arching over all the commissural apparatus of the hemispheres. 

 The concomitant extent of the convolution (hippocampus major) 

 is shown in lxx'. pi. vii. figs. 3 (Didelphys) and 4 (Macropus), 

 in the exposure of the ventricle from the outer side. In 

 Didelphys, fig. 73, the surface of the hemisphere above the 

 fissure is feebly impressed by blood-vessels ; in Thylacinus there 

 is a short fissure above the back part of the hippocampal 



one ; in Phascolomys and Macropus there is also an anterior one 



which bends or bifurcates at its fore part. 1 These fissures mark 



the level of the roof of the 



Lateral ventricle ; the surface 



below forming the thin mesial 



wall of the cavity, fig. 75, q, 



which in the higher Pla- 

 cental is defined, as the ' sep- 

 tum lucidum,' by a corpus 



callosum from the part above, 



On the upper surface of the 



hemisphere, in Macropus ma- 

 jor, a longitudinal part of the 



fissure, fig. 74, 8, marks off a 



medial convolution, I, at the 



anterior half, and occasion- 

 ally it is prolonged backward 



by the fissure, 10, as in the 



left hemisphere of fig. 74. 



But there is continued from 



8, in both hemispheres, a 



fissure extending outward, 



which bounds behind the 



part of the hemisphere impressed by the ' sylvian fissure,' 5. The 



1 lxx'. pi vi. figs. 4 and 6, q, q. 



Brain of Macropus major. 



