Fish, Brain of the Fur Seal. 79 



distance from the striatum and arches around the surface of the 

 thalamus in a ventral direction. Caudal to the hippocamp, the 

 cavity is about as largely represented, and in size forms a dispro- 

 portionately large postcornu. Along the mesal wall just caudal 

 to the hippocamp is an ental ridge correlated with an ectal de- 

 pression — the splenial fissure. This is comparable to the calcar 

 or hippocampus minor of the anthropoid and human brains. It 

 is larger in proportion than either of the above. The splenial 

 in this case for a part of its course at least is, therefore, a total 

 (Wilder) or complete (Cunningham) fissure since the whole 

 thickness of the parietes is involved ; the ental elevation being 

 correlated with the fissural depression. In this specimen of 

 Phoca, then, we have two total fissures, the hippocampal (al- 

 ways) and a portion of the splenial. 



The conditions just described might naturally suggest a 

 homology with the ape and human calcar and that the splenial 

 fissure, in this seal possessing a postcornu, might be homolo- 

 gized with the occipital or calcarine fissure in man. A question 

 might properly arise here as to which fissure it might be homol- 

 ogized with. In the human foetus the occipital is a total fissure, 

 but loses its totality (ental elevation) in the adult. Its position 

 might favor its homology with the splenial, for if the latter 

 were rotated farther caudad it would come to occupy approxi- 

 mately the same position as the occipital. To homologize with 

 the calcarine we would have to imagine a still farther rotation 

 of the splenial. The calcarine is a total fissure throughout life 

 and is the correlative of the calcar. Some doubt may therefore 

 be expressed, assuming the homology to be reasonable, whether 

 this hippocampus minor represents the occipital eminence — a 

 foetal condition in the human brain, or the calcar — a structure 

 persistent in the adult. 



The relative disproportion in the growth of the caudal or 

 occipital portion of the cerebrum may have some bearing in 

 accounting for the presence of the postcornu. Tiedemann in 

 his figure of the lateral ventricle of Phoca gives no indication 

 whatever of a postcornu. 



In CallorJiinus the conditions resemble more closely those 



