114 THE VOYAGE OF H.M.S. CHALLENGER. 



his diagram of the outer surface of the hemisphere (op. cit., fig. 93, p. 118) that he does 

 not consider the interval of separation between the medial and medilateral folds to be 

 as definite as those between the other convolutions on the same surface. Krueg, who has 

 also studied the brain of Phoca vitulina, devotes his description to an account of the 

 fissures, and does not even name the convolutions. It would seem, however, both from 

 his description of the fissures and accompanying figures of the brain, that he only recog- 

 nises three tiers of convolutions on the outer surface of the hemisphere, whilst a well- 

 marked splenial fissure on the inner surface individualises the hippocampal and callosal 

 convolutions. Paul Broca's account of the brain of a Phoca is principally taken up with 

 a description of the great limbic lobe and its relation to the olfactory apparatus. 



In the brain of Phoca vitulina, two specimens of which I have dissected, I found on 

 the outer surface of the hemisphere a distinct fissure of Sylvius, with its Sylvian 

 convolution, the anterior limb of which was narrower than the posterior, and at its com- 

 mencement concealed within the fissure of Sylvius. When this fissure was widely opened 

 out, prolongations of the Sylvian convolution were traced deeply into it, and occupied the 

 position of an insula. Above the Sylvian convolution were a suprasylvian fissure and 

 convolution, the latter of which showed at its summit a disposition to subdivide into 

 two parallel gyri for a short distance. This convolution was bounded above by a 

 lateral fissure, between which and the mesial longitudinal fissure were two slender con- 

 volutions running antero-posteriorly ; the lateral of these was apparently the mediolateral 

 convolution, whilst the medial one bounded the longitudinal fissure and was the sagittal 

 convolution ; as in Owen's figure, however, the fissure which separated the sagittal 

 (medial) from the mediolateral convolution was not continuous, but was bridged by short 

 annectent gyri. As this mediolateral fissure was imperfect and not prolonged far forward 

 in front, the coronal fissure was not continuous with it. In one instance the coronal was 

 prolonged backwards into the lateral fissure, but it might be separated from it by an 

 intermediate bridging convolution. In this region, therefore, the brain of Phoca vitulina 

 closely corresponded in the arrangement both of convolutions and fissures with the 

 Elephant Seal, though in the latter, from its greater size, the convolutions were bigger, 

 also I think more tortuous, and certainly with a greater number of bridging convolutions. 



In the Walrus, again, the four tiers of convolutions were more definitely expressed 

 on the outer surface of the hemisphere, partly owing to the comparative absence of 

 bridging convolutions, and partly because the mediolateral fissure formed so definite a 

 plane of separation between the sagittal and mediolateral convolutions. In this animal 

 also the anterior limb of the Sylvian convolution was narrower, and sunken into the 

 fissure of Sylvius much more than either in Phoca or Macrorhinus, and from Dr. Murie's 

 description and plate of the brain of Otaria jubata (op. cit., fig. 40) it is obvious that 

 a corresponding depression occurred also in that of the Eared Seal. This narrowing and 

 depression were more marked than in Leuret's figures of the Brown Bear, Coati, and 



