34 THE FUR SEALS OF THE PRIBILOF ISLANDS. 



form tbe precoruu aud caudo-lateroventrad to form the medicornu. The striatum is 

 a well-deflued body forming a portion of the floor of the ventricle in the cephalic 

 region. Parallel with the oblique margin of the striatum is the flmbrial margin of 

 the hlppocamp. Between these two margins— the rima (great transverse fissure), the 

 choroid (para) plexus— a continuation of the velum enters the floor of the cavity. 

 The hippocamp pursues its usual curved direction iu the medicornu. 



In Phoca the lateral ventricle is relatively very much larger than in the bear, and 

 the parts present (piite different relations to each other. The striatum is the same 

 as in the bear; along its margin is a well-developed plexus, but between this and the 

 flmbrial edge of the hippocamp there is an area equally as large as the striatum ; this is 

 the optic thalamus, but that portion of it represented in the floor of the cavity presents 

 the same general appearance as to its surface (eudymal) as do the other parts. The 

 supposed delicate endymal membrane extending from the plexus to the fimbria has been 

 designated as the paratela by Wilder. The hippocamp, then, is removed some little 

 distance from the striatum and arches around the surftice of the thalamus in a ventral 

 direction. Caudal to the hippocamp, the cavity is about as largely represented, aud in 

 size forms a disproportionately large postcornu. Along the mesal wall Just caudal to 

 the hippocamp is an ental ridge correlated with an ectal depression— the splenial 

 fissure. This is comparable to the calcar or hippocampus minor of the anthroi)oid and 

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

 in this case, tor a part of its ccmrse, 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 J'hoca, 

 then, we have two total fissures— the hippocampal (always) 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 homologized with the occipital or calcarine fissure in man. A question might 

 properly arise here as to which fissure it might be homologized with. In the human 

 f(etus 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 approximately the same position as the 

 occipital. To homologize with the calcarine we would have to imagine a still fiirther 

 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 iu the human brain— or the calcar, a structure persistent 



ill 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 Plioca, gives no indication whatever 

 of a postcornu. 



In CaUorhinus the conditions resemble more closely those in the bear; the rima is 

 narrow and the thalamus does not appear at all in the floor of the ventricle. A 

 slight caudal spur of the cavity at the medicornu represents the postcornu. The 

 splenial fissure, so to speak, just escapes tbe cavity, lying immediately caudal to it. 



