REPORT ON THE SEALS. 105 



senting the triradiate fissure, was sometimes not furcated, was also present. The gyrus 

 rectus (re) was definitely marked between the olfactory and mesial longitudinal fissures ; 

 between the olfactory fissure and the intraorbital fissure was a well-marked internal 

 supraorbital gyrus (isc), and between the intraorbital and prsesylvian fissures was a 

 relatively broad external supraorbital gyrus (esc). 



The bridging convolutions were not so numerous in the brain of the Walrus as in the 

 Elephant Seal, and the arraugement of the convolutions in four successive tiers, with 

 intermediate sulci, above the Sylvian fissure, was more simple and more easily deter- 

 mined. But it should be stated that the convolutions in opposite hemispheres of the 

 same cerebrum were not perfectly symmetrical, and that one of the larger brains bad 

 more frequently bridging convolutions than the other. The convolutions, lying in rela- 

 tion to the Sylvian fissure, were bounded in front by the pr&sylvian fissure (ps), which, 

 commencing on the supraorbital area, ran outwards and then upwards on the side of 

 the hemisphere to become continuous on the vertex with the lateral or second curved 

 fissure. The external supraorbital gyrus separated it from the intraorbital fissure, whilst 

 between its upper end and the cruciate fissure the anterior part of the mediolateral 

 and the sigmoid convolutions were interposed. The Sylvian convolution (syc) immediately 

 bounded the Sylvian fissure ; its anterior limb consisted of a narrow, tortuous part 

 which was at its commencement so sunk into the Sylvian fissure as to be concealed 

 within it. As it ascended it emerged from the fissure, and formed the immediate 

 boundary of the apex of the fissure, round which it bent, and was continued behind 

 into a broad convolution situated on the surface of the hemisphere, which formed the 

 posterior Up of the Sylvian fissure and consequently the posterior limb of the Sylvian 

 convolution. The broad posterior limb was partially divided into two parallel gyri by a 

 fissure, which in one brain ran almost vertically, in another obliquely. 



Between this Sylvian convolution and the mesial longitudinal fissure three distinct 

 convolutions intervened, which were separated from each other by fissures both on the 

 vertex and anteriorly, though the two uppermost blended with each other posteriorly. 

 The general direction of these convolutions was antero-posterior, but they dipped down- 

 wards towards the under surface of the hemisphere both in front of and behind the 

 Sylvian fissure. The convolution next above the Sylvian convolution was the supra- 

 sylvian convolution (ssc), which was strongly developed and tortuous both in front of and 

 behind the Sylvian convolution ; it showed a tendency, both in front of the Sylvian con- 

 volution and opposite the apex of the fissure of Sylvius, to subdivide into two secondary 

 gyri lying parallel to each other. The Sylvian and suprasylvian convolutions were 

 separated from each other by the suprasylvian fissure (ss), which was partially concealed 

 within the Sylvian fissure anteriorly, owing to the depression of the corresponding- 

 portion of the Sylvian convolution, but it was very distinct on the surface of the hemi- 

 sphere behind, and formed the fissura suprasylvia posterior (ssp). 



(zool. chall. Exp.— part lxviii. — 1S88.) Yyy 1-t 



