NO. 1325. CEREBRAL FISSURES OF THE WALRUS— FISH. 681 



with the medilateral, which at this point has a superficial connection 

 with the cruciate. There is no distinct evidence of a prccruciate 

 upon this heniicerebruni. On this account there is no area that nia}- be 

 correlated in any way with the ''Ursine Lozenge"" thought by Mivart 

 to be of considerable importance in showing a relationship between 

 the seals and the carnivora. In the walrus the cruciate area is cpdte 

 similar to that of Phoca. In Monachus a slight or rudimentar}' 

 "Ursine Lozenge'' may be detected. In Ursus, Callorhinus, and 

 Zalophus the ''lozenge'' is well developed. The sigmoid gvre sur- 

 rounds the frontal portion of the cruciate fissure. 



Suj>erorhital fissure. — This fissure occupies the usual position upon 

 the ventro-iateral surface of the frontal portion of the brain. On the 

 left hemicerebrum it is about 50 millimeters in length. It arises near 

 the rhinal fissure not far from the base of the olfactory peduncle. It 

 curves laterally and then back again toward the mesal aspect so that 

 its termination is covered b}'^ the olfactorj^ bulb. At the lower third 

 of its course it gives off a short and ver}^ superficial lateral branch. 

 On the right hemicerebrum the fissure is very similar to that on the 

 left, except that the lateral branch is much smaller. 



Lateral gyro (Mediolateral convolution of Turner). — In the pres- 

 ent specimen this gyre is not well represented upon the caudal portion 

 of the brain. The short length of the lateral fissure brings about the 

 unusual condition of having the mediolateral fissure form a portion of 

 the boundary of the supersylvian g3're. In this case, therefore, the 

 medilateral gj^re begins well up toward the vertex in a tapering- 

 manner and gradualh' becomes wider as it arches toward the frontal 

 portion of the brain, attaining a width of from 30 to 40 millimeters in 

 its widest parts. Like the supersylvian gyre, it contains a number of 

 minor fissures. 



2lar</!nal or sagittal gyre. — On the left hemicerebrum this is a very 

 narrow gyre and is represented onh' on the caudal portion. It dis- 

 appears at the vertex. This disappearance is due to the fact that the 

 frontal portion of the interrupted medilateral fissure lies upon the 

 mesal aspect. Two or three traces of minor fissures appear in the left 

 saggittal gyre. On the right hemicerebrum the gyre has a fair width 

 in its caudal portion, but liecomes narrower, until at the vertex it 

 almost disappears, but it gradually widens again until in the frontal 

 portion it attains the width of 17 millimeters. On this hemicerebrum 

 there are a number of quite well-developed minor fissures present, 

 which in almost every instance extend over upon the mesal surface. 



Pre and post sylvian areas. — The post sylvian region comprises the 

 unusually wide caudal limb of the sylvian gyre, the supersylvian gyre, 

 and the narrow marginal (or sagittal) gyre. The presylvian region 

 includes the very narrow frontal limb of the sylvian gyre, and the 

 wide frontal limbs of the supers3dvian and medilateral gyres. On the 

 Proc. N. M. vol. xxvi— 02 — -46 



