38 THE FUR SEALS OF THE PRIBILOF ISLANDS. 



The questiou arises if the fissure concealed in the Sylvian may not be the equivalent 

 of the anterior-postica of Krueg, and the two remaining visible on the cranial surface 

 the supersylvian and lateral. 



The mediolateral of other authors does not attain the size nor continued length in 

 the frontal direction as ascribed to the mediolateral by Turner. 



In a specimen of Ursns americanus, I bad the good fortune to discover a stage one 

 step beyond that described by Professor Turner. On opening the Sylvian tissure I 

 found in its caudal wall a completely submerged fissure, with a remnant of the Syl- 

 vian gyre, which might possibly be mistaken for the insula. A true insula, although 

 small, is present. This submerged fissure I take to be the disappearing vestige of the 

 ectosylvian (Owen) or anterior-postica (Krueg). 



It would seem, then, that the condition thus described in the polar bear and 

 American bear would represent the method of disappearance, rather than the ai)pear- 

 ance, of the first circumsylvian arch, and prepare us for the conditions that we find in 

 the sea lion {Zalo2)Jnis) and the seals [Phoca and Callorhinus). 



In tlie sea lion the conditions regarding the frontal portion of the Sylvian gyre 

 are intermediate between the bears and seals. The presupersylvian fissure approaches 

 very closely to the Sylvian fissure, and the intervening portion of the Sylvian gyre, 

 besides being narrower than in the bears, has also sunk slightly lower than the adja- 

 cent surfaces, as if prophesying the conditions found in the seals. 



In the seals there appears to bo some evidence, if the interpretation as to the 

 frontal portion of the supersylvian fissure be correct, that after breaking up into 

 branches, with, perhaps, some disconnection of its parts, it shows a tendency to follow 

 the example of the anterior-postica fissure, because in Flioca, at least, the supersyl- 

 vian bifurcates a little beyond the free end of the Sylvian, one branch forming a well- 

 defined arch around it, the other branch passing on in the frontal region. The 

 branch, however, which forms the arch is not a long one, but it extends to and sixper- 

 ficially connects with a vertical fissure which, for half its distance, is submerged in 

 the frontal wall of the Sylvian, and croi)s out again on the ventral aspect of the brain. 

 This condition holds for both heraicerebrums of Phoca. CaUorMnns throws a little 

 light on this matter. In the right hemicerebrum the supersylvian is clearly continu- 

 ous with the vertical fissure submerged in the frontal wall of the Sylvian, but gives oft" 

 a very short frontal branch. Superficially it is continuous with the postsupersylvian, 

 but a shallow at this point indicates a partial separation. The direct continnity in the 

 depth of the supersylvian with the vertical fissure would seem to i)oint to the fact that 

 the latter, after all, was nothing more than the frontal po;"tion of the supersylvian, 

 namely, the presnpersylvian. 



In the left hemicerebrum the parts sire a little more complicated. The postsuper- 

 sylvian is entirely separated, the supersylvian is entirely distinct from the frontal 

 portion, and is quite irregular and branching in its course, but mainly vertical in its 

 direction. 



Thus, taking the canine brain as exemplifying a simple fissural i)attern, ait<l i)ass- 

 ing through the Felidfv ami Ursidw and sea lion to the seals, where the fissures are 

 more numerous and complicated by the presence of branches of considerable size, and 

 more or less disconnection of some of the principal fissures, we may arrive at some 

 understanding of the relationship and Qhanges eftected in i)assing from simple to 

 complex conditions. 



