26 THE FUR SEALS OF THE PRIBILOF ISLANDS. 



The gyre, bounded by the hiteral and supersylvian fissures aud their parts, is 

 indented by numerous branches originating from the above named fissures. There 

 are also occasionally independent minor fissures present in this gyre. 



The ectolateral Jissiire. — The ectolaterai on the right lieiiiicerebrum is a distinct 

 fissure. It is present on the ventral surface near the termination of the postrhiual. 

 It then proceeds dorsocaudally, parallel with the postsupersylvian and for about the 

 same distance. On the left side it is a shorter fissure and superficially is continuous 

 with the dorsal portion of tlie iwstsupersylvian, but a shallow separates a deeper 

 connection. In the adult Callorliinus the fissure is as on the right hemicerebrum of 

 the pup. 



The coronal fissure is about 3 centimeters in length and extends, except for a 

 slight caudal (ionvexity, in an almost vertical (dorso-caudal) direction. Its greatest 

 depth is 8 millimeters. On the left hemicerebrum it gives off a slight spur, pointing 

 toward the Sylvian. In CaUorhinus it represents, perhaps, the least complicated 

 fissure in the brain. 



The cruciate fissure is not at all represented upon the mesal surface of the brain. 

 It is seen best from a dorsal view. It arises at the margin of the intercerebral cleft. 

 It arches in an obliquely cephalo lateral direction. From the cephalic extremity of 

 the cruciate, at a deptii of 15 millimeters, there passes ofi" another fissure, which 

 Krueg has represented as the precruciate in cei'tain caruivora, nearly to the mesal 

 margin, just dorsal to the olfactory bulb. The depth of these fissures at their 

 junction is from 12 to 15 millimeters. Jietween these fissures and the intercerebral 

 cleft there is a triangnlar-shai)ed area, to which Mivart has applied the name of 

 "ursine lozenge" (Turner), thouglit by Mivart to be of considerable significance. 

 Just caudal to the cruciate fissure is a small fissure corresponding to the postcruciate 

 of Krueg. On the left hemicerebrum it is triradiate; on the right it is straight. 



The superorhital fissure has no connection with the rhinal. Its length is 25 

 millimeters and its depth 8 to 10 millimeters. It has a slight lateral convexity, but 

 has no branches. 



The medilateral fissure. — The name of this fissure is particularly api)ropriate in 

 CaUorhinus. Not only is it on the mesal side of the lateral, but for a portion of its 

 course is actually on the mesal asi)ect of the brain. It curves anmud the caudal 

 margin of the hemicerebrum Just on the verge of the margin. Between the lateral 

 aud medilateral fissures there is a gyre averaging about 15 millimeters in width, in 

 which there are two or thiee secondary fissures, which would seem to indicate an 

 attempt at the division of this gyre into two. 



MESAL ASPECT. 



The callosal fissure presents no marked peculiarity except ui)on the left hemi- 

 cerebrum, where, instead of continuing around the genu of the callosum, it proceeds 

 toward the dorsal margin, or is continuous, with a fissure coming from this margin. 

 On neither hemicerebrum is there any appearance of a fissure immediately surround- 

 ing the genu. The hippocampal fissure occupies its usual position, arching from the 

 splenium around the oi)tic thalamus to the lip of the ])yriform or temporal lobe. 



The sphnial fissure. — On the right hemicerebrum this fissure, if prolonged upon 

 the dorsal aspect, would be continuous with the cruciate, it is sei)arated by a gyre 

 4 millimeters in width. The fissure passes ventro-caudally and a little beyond the 



