Fish, Drain of the Fur Seal. 59 



depths. In order to obtain the desired results, after photo- 

 graphing the dorsal and ventral surfaces of the entire brain, it 

 was cut across and the crura cerebri or mesencephal, and the 

 cerebellum and oblongata separated. The cerebrum was then 

 divided by a section along the median line, separating it as 

 nearly as possible into two equal halves. 



Removal of dura. The falx showed an interesting devel- 

 opment, its frontal portion, especially in the region of the 

 olfactory bulbs, being of considerable depth, then becoming very 

 shallow along the middle of the length of the cerebrum and be- 

 coming very deep again in the intercerebral cleft in the caudal 

 region of the cerebrum. A distinct longitudinal venous sinus 

 as in the human brain is not present ; but in place of it is a vein 

 of some size lying to the right of the ( intercerebral ) cleft and 

 receiving the contents of the dorsal cerebral veins. In connec- 

 tion with the weak development of the falx along the middle of 

 its length, there was noticed an interdigitation of the gyres of 

 the mesal surface of the hemicerebrums in this region. This 

 intimate overlapping of the gyres on the mesal surfaces of the 

 two hemicerebrums is possibly correlated with the deficiency of 

 growth of the falx here and may serve, in a measure, to increase 

 the firmness of the union of this region and prevent any undue 

 strain upon the callosum which lies some little distance from 

 the dorsal surface of the cerebrum. 



This interdigitation of the mesal gyres is also present in the 

 sheep where the falx is also deficiently developed. If the hem- 

 icerebrums be divided with a sharp knife without first separating 

 the pial adhesion of the gyres, the gyres will be cut. An artifact 

 of this nature has, indeed, been mistaken by one writer in an 

 article on Phoca, for the cut surface of a bundle of fibers dorsal 

 to and larger than the callosum and designated by him as the 

 commissura suprema. 



The tentorium in Callorhinus is very strongly developed, 

 apparently extending the whole depth of the transverse arch- 

 like cleft between the cerebrum and cerebellum. The tough 

 fibrous tissue of the tentorium is, moreover, very noticeably re- 

 inforced by the presence of osseous tissue. Where the falx 



