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



from the figures published by Tiedemann and Leuret. The crucial fissure was at the 

 anterior end of the hemisphere, and about 14 mm. in its transverse diameter, and had 

 the usual relation to the sigmoid gyrus, with which the sagittal convolution was con- 

 tinuous. No prsecruciate fissure could be seen on the cranial surface of the cerebrum, 

 but, when the hemispheres were separated from each other, a short fissure was recognised 

 passing downwards from the crucial fissure, which apparently was the prascruciate fissure, 

 whilst the short convolution which it differentiated represented the ursine lozenge, 

 situated as Mivart has stated entirely on the mesial surface of the hemisphere. The 

 prorean convolution was continued into the gyrus rectus. 1 



The convolutions and sulci on the inner and tentorial surface of the hemisphere of 

 Macrorhinus corresponded in essential particulars with those of Phoca. Some differences 

 are, however, to be noted. Thus in Macrorhinus the splenial fissure was not continuous 

 with the postrhinal fissure, neither was it bridged across superficially by a retrolimbic 

 pli-de-passage, though there was a short gyrus projecting backwards from the hippo- 

 campal convolution which may represent it. In both hemispheres the splenial fissure 

 was continued into the crucial fissure ; the demarcation of the splenial from the sagittal 

 convolution by a continuous antero-posterior suprasplenial fissure was less marked in 

 Macrorhinus than in Phoca. 



In the Walrus, also, the splenial and postrhinal fissures were not continuous with 

 each other. In one brain (a) (PI. IX. fig. 3) there was no retrolimbic bridging convolu- 

 tion, which was present however in both hemispheres of another specimen, and in one of 

 these hemispheres was represented by two convolutions. In two brains the splenial 

 fissure joined anteriorly the crucial fissure. The definition of the suprasplenial convolu- 

 tion and fissure varied in opposite hemispheres. Both brains possessed postero-horizontal 

 and postsplenial fissures and a splenial convolution. The olfactory peduncle and bulb 

 were larger than in the Phocidse. 



In Otaria jubata, if I may judge from Dr. Murie's drawings of the brain of that 

 animal, the postrhinal and splenial fissures were not continuous with each other ; the 

 splenial fissure was bridged by a retrolimbic convolution ; the splenial fissure was not 

 prolonged directly into the crucial fissure ; the suprasplenial convolution and fissure were 

 not sharply differentiated ; the olfactor}^ apparatus was more like in size the same parts 

 in the Walrus than in the Phocidce. 



Dr. St. George Mivart has recently introduced into the study of the brain in the 

 Carnivora and Pinnipedia the consideration of the area which he has named the Ursine 

 lozenge, and has pointed out that it constitutes a well-marked feature in the anterior part 

 of the dorsal surface of the cerebrum of Otaria gillespii. I have already stated that, in 

 both Phoca and Macrorhinus, but especially in the former, this area is rudimentary, and 

 concealed in the mesial fissure of the cerebrum. In the Walrus, again, the single con- 



1 Theodor's essay on the brain of Phoca vitulina did not come into my hands until after this Report was in proof. 



