TROSENCErilALON OF MAMMALS. 



121 



primary fissures: ecto- and ento-rhinal, hippocampal, fig. 99, 4, 

 sylvian, fig. 106, 5, callosal, fig. 99, 7, lambdoidal, figs. 96, 101, 13, 

 and entolambdoidal, fig. 99, 13', supersylvian, fig. 101, 8, and 

 lateral or mcdilateral, ib. 10. In Tragulus, fig. 101, this is slightly 

 indicated: in Hyrax, fig. 96, 10, it is longitudinal but short. 

 Hyrax also shows a frontal fissure, ib. n, and the postsylvian 

 fissure, ib. 9. But the chief difference is seen in the course of the 

 fissure 13, which, impressing the inner and posterior side of the 

 hemisphere at fig. 99, 13', converges toward its fellow in Tragulus, 

 but diverges as it advances, in Hyrax, and extends beyond the 

 frontal fissure, H, simulating the longitudinal one in the same 

 part of the brain in the Agouti, Sloth, and Aye-aye. Its fore-part 

 runs into the superfrontal fold, n, or divides it from the sub- 

 frontal, n'". We may trace the homologue of 13, interrupted by 

 secondary fissures, marking off a less elongated mesial tract of the 

 brain, in the Horse, fig. 97, and Rhinoceros, fig. 98. The primary 



Hyrax. 



Rhinoceros. 

 Inner surface of hemisphere. 



fissures, in Hyrax, define the following folds : viz., sylvian, figs. 

 96, 106, e, e , postsylvian, ib. f, part of supersylvian, ib. g, 

 blending with the medilateral or medial, Z, the entolambdoidal, 

 fig. 99, p' , rising to the upper surface, confluent with the tract 

 representing the superfrontal, w x ; in like manner the sylvian, 

 fig. 96, e', blends with the subfrontal tract, ib. n' h '. On the inner 

 surface of the hemisphere, the hippocampal fold, fig. 99, a, the cal- 

 losal, k, and entolambdoidal, p' ', are defined ; with an indication of 

 a falcial fold, 0". The ectorhinal fissure at the base of the brain 

 bounds the outer side of the rhin encephalic part, and divides it 

 from the more exterior part of the ( natiform protuberance.' 



In the larger Perissodactyles, the expansion of the hinder part 

 of the hemispheres is attended with a greater recession of the 

 supersylvian fissures, fig. 97, Horse, and fig. 98, Rhinoceros, 8, 

 outward and backward ; they also become deeper, more continuous, 

 and more wavy: the lateral or medilateral fissure, 10, is now deve- 

 loped to a similar extent, running parallel with 13 and 8, and with 

 more depth in Rhinoceros, fig. 98, than in Equus, fig. 97. Both 



