PROSENCEPHALON OF MAMMALS. 109 



the hippocampal fissure in each, which accordingly passes above 

 the transverse commissural system (' lyra of fornix') in the 

 Marsupial, and beneath the abruptly superadded e corpus cal- 

 losum ' in the placental Insectivore. In a similar section of 

 the brain of the Squirrel 1 the corpus callosum is of greater 

 relative extent, as it is in all Rodents as contrasted with In- 

 sectivores. Concomitantly with the appearance of the new series 

 of transverse fibres bringing the hemispheres into communica- 

 tion above their ventricles, the anterior commissure is diminished 

 in size. 



Notwithstanding this difference in the kind and arrangement of 

 the transverse connecting fibres of the hemispheres, these do not 

 present a corresponding rise of developement. In the snouted 

 Shrews of Africa the brain, fig. 76, offers outwardly as low a 

 condition as in the Opossum or Dasyure. All the four primary 

 segments are in view ; the epencephalon, c, mesencephalon, o, 

 prosencephalon, p, and rhinencephalon, Pv, suc- 

 ceed each other longitudinally from behind 

 forward, as in Reptilia. The multiplication 

 of grey and white matter above the medulla 

 oblongata mainly distinguishes the brain of 

 the active Shrew from that of the slow Tor- 

 toise, fig. 45 ; and the lateral lobes of the 

 cerebellum carry appendages, as in the Opos- 

 sum. The anterior bigeminal bodies, O, much 

 exceed the posterior ones in size. A feeble 

 and interrupted indication of the medilateral 

 longitudinal fissure marks the upper surface 

 of the hemispheres. These are much contracted anteriorly. A 

 short callosal fissure is added to the hippocampal one on the 

 inner surface of the hemisphere. The rhinencephala are long, 

 large, and pyriform. In the Hedgehog (Erinaceus) the ectorhinal 

 fissure is apparent in the upper view of the brain through the 

 great relative size of the crura rhinencephali. 



The Bats resemble the terrestrial Insectivora in their cerebral 

 surface, as do also the smaller Rodents. In some of the larger 

 ones, Agouti, e. g. (vol. ii. p. 270, fig. 146), the medilateral 

 fold is better defined : but the Beaver shows no trace of this, 

 although the hemispheres are broader anteriorly : they are more 

 expanded here in the equally smooth cerebrum of the Porcu- 

 pine, fig. 77. The Rodents show some variety in the shape of 



1 xx. no. 1323 H. 



