110 



ANATOMY OF VERTEBRATES. 



77 



the cerebrum. In the Leporidce, fig. 79, a, it is lozenge-shaped, 

 with the anterior borders longer, and converging to a narrower 

 (though obtuse) apex, than the posterior ones. In the Pacas 

 the cerebrum is broader, with both ends more obtuse and larger, 

 and the hinder third is broader. In Castor, fig. 78, it pre- 

 sents a full ovate figure. In 

 Hystrix, fig. 77, it is subquadrate, 

 through increasing breadth of the 

 fore part. On the medial surface 

 of the hemisphere the ' hippocampal 

 fissure ' is confined to the hinder 

 half; the ' callosal fissure ' is super- 

 added, commencing at the ( sple- 

 nium' or posterior genu of the 

 great commissure, and running 

 along its upper surface to the an- 

 terior genu ; it is shallow, but now 

 defines the true ' labium cerebri.' 

 On the under surface the ectorhinal 

 fissure, figs. 83 and 84, 2, has the 

 same extent as in the Wombat ; it 

 diverges, as it recedes, further from 

 its fellow, in fig. 83, through the 

 greater breadth of the basirhinal protuberances, h. A few 

 short fissures rise from its anterior half a little way upon the 

 hemisphere in the Cavies, as in the Wombat. In the Por- 

 cupine the sylvian fissure, figs. 77, 84, 5, is well marked, 

 though short, and there is a feeble indication of the ' coronal 

 fissure,' 12. 



In the smaller and especially the insectivorous Bruta the 

 brain presents the lissencephalous type, having smooth, low, 

 triangular hemispheres, leaving the mesencephalon as well as 

 epencephalon in view posteriorly. Dasypus has the fore part 

 less contracted than Myrmecophaga, at least than M. didactyla. 

 In Bradypus the anterior expansion gives an ovate form to the 

 hemispheres ; and now, besides the hippocampal, callosal, ecto- 

 rhinal, and sylvian fissures, the upper surface shows the medi- 

 lateral, suprasylvian, and frontal ones. The medilateral bends 

 outward anteriorly, defining the ' anterior lobe ' impressed by 

 the short angular or triradiate c frontal ' fissure. The above 

 fissures mark out a medial, lateral, sylvian, postfrontal, and 

 prefrontal convolutions. On the inner surface a supercallosal 



Upper surface of the brain of the Porcupine. 



