080 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvi. 



In some cases it lies exactly along this margin (Monachus); in other 

 cases it lies partly upon the lateral and partly upon the niesal surface 

 (Callorhinus), or, as in Zalophus, it may be better seen upon the 

 mesal aspect. In the walrus it is better seen upon the lateral aspect. 

 On the left hemicerel)rum it arches forward to the vertex, but does 

 not quite reach the mesal margin. At this point another fissure 55 

 millimeters in length continues forward from the mesal margin in the 

 same direction that the medilateral would take if it were longer. The 

 inference is that it is properly a portion of the medilateral cut off by 

 a narrow isthmus of the cortex. 



Another fissure, 70 millimeters in length, separated (from the tissure 

 just described) by an isthnuis 13 millimeters, continues forward and 

 downward ii[)on the mesal aspect as far as the cruciate tissure, and has 

 a slight su))('rticial connection with it. The appearances indicate that 

 th(> medilateral on the left h(nnicerel)rum is divided into three i)or- 

 tions — the caudal portion, 1)0 millimeters long, located on the lateral 

 surface; the middle portion, 55 millimeters, lying in the margin ])etween 

 the mesal and lateral surfaces; and the cephalic portion, 7<> millimeters 

 long. l3'ing entirely upon the mesal aspect. On the right hemicere- 

 bruni the medilateral lies upon the dorso-lateral aspect and begins far 

 down on the caudal portion of the hemicerebrum, arching upward and 

 forward as far as the vertex. It lies very close to the meso-lateral 

 margin but recedes from it gradually until the vertex is reached. It 

 is interrupted at this point by a cortical isthmus 5 millimeters wide. 

 The cephalic portion of the tissure begins a little mesal to the termi- 

 nation of the caudal portion. As it arches forward and downward it 

 recedes from the mesal margin and extends to a point a little lieyond 

 the cruciate. Turner " figures the medilateral upon the lateral aspect 

 of both hemicerebrums of the walrus as a long uninterrupted fissure, 

 quite close to the niesal margin caudally but receding from it as it 

 arches downward and forw^ard. In my specimen the medilateral is 

 divided into two portions on the right hemicerebrum, both lying on the 

 dorsolateral surface. On the left hemicerebrum it is divided into three 

 portions and the frontal portion lies upon the mesal surface. If it 

 were a continuous fissure it would be by far the longest tissure of the 

 brain. 



Cruciate ^fixsurc. — This fissure just cuts through the mesal margin 

 and extends only 12 millimeters u})on the frontal portion of the lateral 

 surface. On the left hemicerebrum there is a postcruciate fissure, 

 triradiate in form, represented. The precruciate is not well repre- 

 sented except by a short fissure lying in the mesal margin, which 

 fuses into the cruciate. On the right hemicerebrum the postcruciate 

 is not represented as a distinct fissure. It may have become fused 



« Report on the seals collected during the voyage of H. M. S. Challenger, in the 

 years 1873-1876, Zoology, XXVI, 1888, pp. 89-134. 



