BRAIN OF FUR SEAL. 25 



the surface is entirely smooth. In the caudal wall of the Sylvian is a well-marked 

 subfissure. It separates a portion of the concealed cortex, forming a subgyre, which 

 from its size and position might be easily mistaken for the insula. The appearances 

 would suggest that the subtissure is the postica and the subgyre a remnant of the 

 Sylvian gyre. 



The supersylvian fissure shows some variation on the two sides. It presents the 

 usual arrangement on the right hemicerebruni, forming, superficially at least, a com- 

 plete arch around the Sylvian. The presence of a shallow and a slight bifurcation 

 near the level of the free end of the Sylvian indicates the separation of a postsuper- 

 sylvian fissure — postsylvian of other writers. (PI. I., fig. 4.) The supersylvian curves 

 around the free end of the Sylvian at rather a sharp angle and soon apparently enters 

 the Sylvian, but in reality is submerged in its cephalic wall. A very short cephalic 

 branch is given off toward the ansate fissure before the supersylvian enters the 

 Sylvian. On the left hemicerebruni there are three distinct portions; the postsuper- 

 sylvian has a slightly more oblique dorso-caudal course, the supersylvian proper is 

 quite branching and more inclined to a vertical than horizontal course. One of its 

 branches appears to enter the Sylvian from behind, but a shallow shuts off any deep 

 connection. The frontal portion appears as a surface fissure for only one-third of its 

 course, then, as on the other side, it becomes submerged in the Sylvian. As this 

 portion bears much the same relation to the supersylvian as the postsupersylvian 

 whether they be disconnected or not, the frontal portion will be designated as the 

 presupersylviau fissure. In a second specimen of the brain of an adult Callorhinus, 

 kindly loaned me by Mr. True, the executive curator of the United States National 

 Museum, both hemicerebrums showed a distinct separation of the postsupersylvian, 

 more pronounced than on the right hemicerebrum of the pup ; but there was no sepa- 

 ration nor distinct appearance of a shallow indicating an independent presupersylviau 

 as in the left hemicerebrum of the pup. In the adult, as in the pup, each supersylvian 

 gave off a short cephalic branch before entering the Sylvian. 



The lateral fissure, on account of the breadth of the brain, does not show in its 

 entirety upon the lateral aspect. It is 12 centimeters long, by far the longest fissure, 

 and is seen for a short portion of its course upon the ventral aspect, extending, on the 

 left hemicerebrum, to within 5 millimeters of the ventral portion of the postsuper- 

 sylvian. It lies in this region just in advance of the margin of the cleft between the 

 cerebrum and cerebellum. It then arches caudo-dorsally approximately parallel with 

 the hemicerebral margin, but receding from it until it fully reaches the dorsal surface, 

 then approaching to within 8 or 9 millimeters of the intercerebral cleft, it continues 

 its arched course in a cephalo- ventral direction, approaching to within 5 millimeters of 

 the presupersylviau fissure at about the level where the latter becomes submerged into 

 the sylvian. 



The lateral is a deep fissure, and no distinct evidence of shallows could be detected 

 along its course, although in certain places the presence of submerged buttresses 

 interfered to some extent with the soundings, the average depth being from 10 to 13 

 millimeters. The cephalic extremity of the fissure terminates in a fork, more marked 

 upon the left hemicerebrum than on the right. Does this widely forked terminus 

 represent the ansate fissure? It has the same appearance and relation to the lateral 

 as seen in the cat, and, provisionally, it is here so designated. 



