(NO. 1325. CEREBRAL FISSURES OF THE WALRUS— FISH. 677 



same form and direction as those in the eephalic wall. One striking 

 exception with regard to the direction of the siihmorg-ed Hssures I have 

 called attention to in a preceding paper." This fissure I have compared 

 with the postica. It corresponds to a fissure of the same name in the 

 feline brain, but differs in this instance, in that it is submerged in the 

 sylvian. Ursus, Zalophus, Callorhinus, and Fhoca also show this pecu- 

 liarity. The postica differs from the other submerged fissures of the 

 sylvian by extending in a direction at nearly right angles to them, that 

 is, dorso-ventrally, paralleling approximateh" for a short distance that 

 of the sylvian itself. 



The true insula is but scarcely developed in the walrus, and api)ears 

 merel}" as a slight elevation in the })ottom of the sylvian fissure. The 

 submerged gyre formed by the postica fissure may easily b(^ mistaken 

 for a well-developed insula, as it has approximately a suitable location. 

 Whether the submerged gyre later forms a closer relationship with the 

 true insula, and is the precursor of the more complicated insula found 

 in the higher forms, is a question that can not be answered here. 

 Ziehen^ describes in the walrus a well-developed insula divided by fis- 

 sures into three gyres. 



Supersijhvdn fissure. — This fissure arches around the distal end of 

 the sylvian in the usual way. Its frontal portion — the presupersyl- 

 vian— passes close to the sylvian, converging gradually until near the 

 base of the sylvian the presupersylvian becomes a submerged fissure, 

 cropping out later to a slight extent upon the ventral surface. There 

 is no evidence of a shallow or vadum indicating a separation of the 

 presupersylvian and the supersylvian proper, as sometimes occurs in 

 the fur seal. In Phoca the two fissures are entirely distinct and are 

 without evidence even of a superficial connection. In the walrus the 

 two fissures are continuous w^ith each other, as in th(^ dog and bear, 

 and the differentiation is therefore an arbitrary oni\ 



Perhaps the most puzzling fetiture regarding the fissures of this 

 specimen of the walrus brain is the relationship of the su[)ersylvian 

 with the postsupersylvian. The conditions appear very nuich as in 

 the brain of the sea lion (Zalophus). The apparent postsupersylvian 

 is four times as far removed from the sylvian as is the presupersylvian. 

 In this wide area between the sylvian and postsupersylvian Zalophus 

 shows a few minor fissures extending horizontally. In the walrus 

 there are also minor fissures present in this area but also a fairly well- 

 developed fissure nearly vertical in its direction, almost connecting 

 with the supersylvian on the right hemicerebrum, but totally discon- 

 nected on the left hemicerebrum. This vertical fissure se^'Uis to be too 

 well developed to be classed with the minor fissures and its position 

 and relations suggest the possibility of its being a poorly developed 



"Fish, The Brain of the Fur Seal {Callorhhms nninus). Report of the Fur Seal 

 Investigation, 1896-97, pt. 3, pp. 21-40. 



& Anatomischer Anzeiger, V, 1890, i)p. 092-709. 



