Fish, Brain of the Ftir Seal. 8i 



the correlation of the splenial fissure with the calcar in these 

 aquatic forms. This fact, even if it be of no direct use for ho- 

 mology, is, at least, interesting. 



MONACHUS TROPICALIS. 



In August, 1897, I was fortunate to obtain through the 

 courtesy of Dr. A. H. Hassall, Washington, D. C, two brains, 

 from male and female specimens of the West Indian Seal Mon- 

 achus tropicalis. They arrived at an exceedingly opportune 

 time for comparison with the other brains dealt with in this ar- 

 ticle. A study of their form and fissural relations throw much 

 light on some of the points which seemed quite aberrant in 

 Phoca when compared with CallorJiinus alone. 



The general form of the brain would suggest a position in- 

 termediate between the fur seal and Phoca particularly in the 

 frontal region which is somewhat foreshortened and broader than 

 in CallovJiinus. 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. In all four hemicerebrums, this fissure 

 sends a branch to the surface, thus appearing superficially as a 

 branch of the Sylvian. The postica is less easily distinguished 

 in MonacJius than in any of the other forms, as it is submerged 

 practically to the bottom of the Sylvian fissure. In CallorJiinus 

 there is a branch corresponding to that of Monachus but it does 

 not extend deeply enough to connect with the postica. 



The postrhinal appears as the merest trace of a fissure and 

 has a very superficial connection with the postica. 



The Sylvian fissure. It is in the Sylvian region that we 

 get numerous clues to the intermediate position of Monachus. 

 In the brain of the female the Sylvian has practically the same 

 direction as in Callorhinus. In the male, the true Sylvian really 

 branches cephalad, although there is a superficial extension in 



