CEREBRAL FISSURES OF THE WALRUS— FISH. 679 



that its cephalic portion is much better developed than the caudal. 

 Ill Ursus, Callorhinus, Phoca. and Moiuichus. on the other hand, the 

 lateral is the long-est fissure of the brain. 



Ansate fissure. — No distinct line of separation exists in the walrus 

 between the ansate and the lateral, and the separation into individual 

 fissures is therefore an ar])itrary one. A like condition exists in the 

 sea lion, bear, and dog. In Phoca, Callorhinus, Monachus, and the 

 cat some differentiation exists, 



(oro/ial fissnrf. — In the left hemicerebrum of the wah-u.s there is 

 no line of demarcation between the coronal, ansate, and lateral fissures, 

 and the three together appear as a long, continuous fissure reaching 

 over upon the ventral aspect. On the right hemicerebrum the coronal 

 is an independent fissure, being separated from the ansate by a pU de 

 jyasxage or isthmus H millimeters in width. On each hemisphere the 

 extent of the coronal upon the ventral aspect is ((uite remarkable. 

 It reaches nearly to the rhinal fissure just in front of the sylvian. 

 In the bear and sea lion the relation of the coronal, ansate. and lateral 

 fissures is quite similar to that of the walrus. 



Supersylvian <jyre. — This gyre surrounds the sylvian gyre and is 

 bounded on the one side by the supersylvian and pre- and post- 

 supersylvian fissures; on the other side by the coronal, ansate, and 

 lateral fissures, and caudo-ventrally l)v a fissure whicii corresponds, in 

 position, to the medilateral fissure. The frontal and dorsal portions 

 of this gyre are well developed, averaging 30 millimeters in width. 

 Numerous minor fissures, having a direction, in general, perpendicu- 

 lar to the l)Oundary fissures, break up the supersylvian gyre into a 

 number of secondary gyres. The caudal liml) of the supersylvian 

 tapers until it reaches a width of only about :20 millimeters, this con- 

 dition probably being due to the extraordinary width (40 millimeters) 

 of the sylvian gyre in this region. 



Ectolateral fissure. — This fissure has already been discussed in con- 

 nection with the postsupersylvian fissure. In the allied forms studied 

 the ectolateral may or may not extend over to the ventral surface 

 of the brain. It ma}' or may not coimect either with the lateral or 

 postsupersylvian, or lie between the two fissures. The fact that the 

 postsupersylvian does not as a rule reach to any extent upon the 

 ventral aspect makes it seem probable that if the ectolateral is at all 

 represented upon the walrus brain it has fused with the postsuper- 

 sylvian. On the left hemicerel)rum it reaches nearly to the post- 

 rhinal fissure. On the right hemicere])rum it is shorter and a minor 

 fissure intervenes, 



Mtdildtend fixsure. — In some foi'ms the name is ])arti('ularly appro- 

 priate if it has any connection with the relation of the fissure to the 

 median and lateral aspects of tlu^ hemicerebnun. Its situation is 

 never very far from the edge or margin separating these two aspects. 



