BRAIN OF MONACHUS. 35 



In the walrus, Turner ^ represents a dissection of this cavity, but shows no indi- 

 ca,tion of a postcornu; but in the text he states: " Where the cavity of the ventricle 

 curved downward and outward into the horn, an indication of a recess was seen in its 

 posterior horn, but it did not amount to a cornu, and there was no elevation which 

 could be called a hippocampus minor. " 



Murie,^ on the form and structure of the manatee, figures a well-developed 

 postcornu. He states that "there is an undoubted posterior cornu, a fully developed 

 hippocampus minor, and an eminence I am inclined to recognize as eminentia collat- 

 eralis." The same author, on the anatomy of the sea lion Otariajuhata figures a more 

 extensive postcornu tli an is represented in the manatee, anddescribes it as " stretching 

 backward and outward with a very regular sweeping arch, and goes well back into the 

 occipital lobe, terminating in a shallow, tapering extremity. The eminentia collat- 

 eralis is not distinctly defined, but what appears to represent the outwardly bulging 

 hippocampus minor has a length of 0.7 of an inch, and at widest is 0.3 to 0.4 broad." 



Wilder, in the Anatomical Technology, in, indicating the lines of inquiry likely to 

 be most productive of results in the homology of the*humau and feline fissures, states 

 that "between the ordinary carnivora and the monkeys are two groups whose brains 

 should be studied with especial care; the seals have a rudimentary postcornu and 

 occipital lobe, and these parts are said to be developed in the Lemurs, which have 

 aflBnities with both the carnivora and the primates." 



In none of the accounts have I seen any direct mention of the correlation of the 

 splenial fissure with the calcar in these aquatic forms. This fact, even if it be of no 

 direct use for homology, is at least interesting. 



MONACHUS TROrtCALIS. 



In August, 1897, 1 was fortunate to obtain, through the courtesy of Dr. A. H. 

 Hassall, Washington, D. 0., two brains from male and female specimens of the West 

 Indian seal {Monachus tropicalis). They arrived at an exceedingly opportune time for 

 comparison with the other brains dealt with in this article. A study of their form 

 and flssural relations throws much light on some of the points which seemed quite 

 aberrant in Phoea when compared with Callorhinus alone. 



The general form of the brain would suggest a position intermediate between the 

 fur seal and Phoca, particularly in the frontal region which is somewhat foreshortened 

 and broader than in Callorhinus. The caudal portion of the cerebrum is much 

 elongated, noticed particularly upon the mesal aspect when measured from the 

 splenium of the callosum, as if, perhaps, to compensate for the foreshortened frontal 

 region. The cerebrum also shows a slightly greater overlapping of the cerebellum. 

 The olfactory bulb and crus resemble the corresponding parts in Phoca, but show a 

 slightly greater development. 



FISSURES. 



Postica.—ln all four hemi cerebrums, this fissure sends a branch to the surface, 

 thus appearing superficially as a branch of the Sylvian. The postica is less 

 easily distinguished in Monachus than in any of the other forms, as it is submerged 

 practically to the bottom of the Sylvian fissure. In Callorhinus there is a branch 



1888, ' Turner, report on the seals collected daring the voyage of H. M. S. Challenger in the years 

 1873-1876. 



3 Loo. cit. 



