24 THE FUR SEALS OF THE PRIBILOF ISLANDS. 



the Sylviau extends in a dorso-cauclal direction, inclining somewhat toward tlie verti- 

 cal. Apparently it terminates in a fork, but when the walls of the fissure are divari- 

 cated it is seen that the cephalic or anterior branch is really another fissure, which, 

 after its superficial union with the Sylvian, becomes a submerged fissure lying just 

 beneath the surface in its cephalic wall and running parallel with it to the base of the 

 brain, but not actually connecting either with the Sylvian or with the rhinal. The 

 Sylvian, on account of the subfissural complication, appears to be a larger fissure than 

 it really is. 



In a former paper' attention was called to the fact that this vertical fissure (super- 

 ficial vertical branch of the Sylvian) had been mistaken for the true Sylvian. Both 

 fissures are well marked and can not be ignored; but it is an unusual circumstance for 

 the Sylvian to assume a strictly vertical position in the adult, and there would, more- 

 over, remain a fissure in the usual situation of the Sylviau unaccounted for. In my 

 former paper I designated this vertical fissure as the anterior of the FeluJa', aiul found 

 at a later date, while consulting Krueg's article,^ that he questioningly represents a 

 similar fissure by the same name in Phoca vituUna. CaIlorhi7ins, while showing this 

 fissure similarly situated, instead of elucidating the complications seems rather to add 

 to them and to suggest a probable doubt as to the correctness of the homology with the 

 anterior fissure. Indeed, the conditions are strongly suggestive of its being nothing 

 more than the frontal portion of the sui)ersylvian fissure. An examination of the 

 brains of certain bears tends to illuminate this view. In the family Ursidcv, as in the 

 Canida', the supersylvian forms a complete arch, the caudal portion being known as 

 the posterior supersylvian (Krueg), or postsylvian (Owen). Tlie frontal portion of 

 this arch varies in its distan(!e from the Sylvian. Occasionally the frontal and caudal 

 portions are about equally distant, but when there is any difference in this distance 

 it appears that the frontal portion approaches more closely to the Sylvian than does 

 the caudal. In Ursus arctos, or the brown bear, Krueg figures the frontal portion 

 of the supersylvian as approximating very closely to the Sylvian. The coiulition in 

 Callorhinus might be considered as a stage .just beyond this. In the brown bear the 

 frontal portion of the supersylvian is still visible upon the lateral surface close to 

 the Sylvian. In the case of the se.al it has passed over the brink, so to speak, and 

 is no longer visible its entire length on the lateral surface. The following diagrams 

 will illustrate the conditions more clearly: 



Bear. 



^:''-r:^'J?v''^ 



Seal. 



A diagrammatic representation of tlio relation of the Sylvian and supersylvian fissures in tlic bear and seal, as if seen in 

 section. PrsB., presupersylvian ; Syl., Sylvian fissure. 



At the bottom of the Sylvian fissure lies the insula, presenting but a slight degree 

 of development. There is a suggestion of a circuminsular fissure, but in other respects 



'1896, P. A. Fisb: A note on the Cerebral Fissuration of the Seal {Phoca vttiilhia). Jour. Conip. 

 Neurol., VI, 15-19. 



-1880, J. Krueg: Ueber die Furchen auf der (iro.sshirnrinde d«r zouoplacentalen Siingethiere. 

 Zeit. f. wiss. Zoologie, XXXIII, 595-672, 5 plates. 



