GENERAL CONSIDERATIONS. 37 



A well-marked Collateral fissure is present and resembles the corresponding 

 fissure in Callorhinns very closely. 



Postcormi. — Perhaps the most important point in connecting Monachus with 

 Phoca is a very well developed postcornu. Callorhinns shows the merest trace of one 

 and in the bears it is absent. In Monachus it does not go so far as in Phoca, a greater 

 portion of the caudal wall being solid. The floor of the postcornu in Monachus is 

 quite distinctly convex. This convexity of the internal surface is found to be cor- 

 related with an external depression, the lower or ventral portion of the splenial fissure. 

 At the more vertical portion of the fissure, namely, opposite the caudal end of the 

 callosum, the splenial fissure loses its totality and becomes an ordinary fissure for the 

 remainder of its upward course. The postcornu stops at the level of the depth of the 

 splenial fissure in the callosal region. We have not, therefore, as in Phoca, a well- 

 developed calcar [Hippocamptis minor). The iaternal convex surface already spoken 

 of iu connection with the ventral portion of the si)lenial fissure offers a suggestion as 

 to the inceiition of the calcar which finds its fulfillment in Phoca. 



GENERAL CONSIDERATIONS. 



The average canine brain, as a matter of convenience, may be accepted as a simple 

 type of a carnivore brain. The fissures are clearly demarcated, and there is an absence 

 of much branching or secondary fissuration. 



Around the Sylvian there are three arched fissures separating the cortical substance 

 into four distinct folds or gyres. In the brain of cats, and occasionally in dogs, we 

 find that the arched fissure nearest the Sylvian is not a complete one; that only the 

 pillars are represented, the keystone being absent. 



In Hyena and Proteles the frontal portion of this arch is wanting (Kraeg), but the 

 caudal portion, fissura postica, is well represented. 



In certain others of the carnivora no trace of the first arch or Sylvian gyre, with 

 its limiting fissure (anterior-postica), is at all present. The first arch with its fissure 

 has disappeared, apparently swallowed up by the Sylvian. There are represented, 

 then, on the lateral aspect only two arched fissures, the supersylviau and lateral and 

 the three gyres which they separate. In those forms iu which only the two arched 

 fissures are present, if the distance from the frontal portion of the supersylviau to the 

 Sylvian be compared with the distance from the latter to the postsupersylvian, it will 

 generally be found to be less in the former, and this becomes much more emphasized 

 in the case of some of the bears, where the frontal portion of an undoubted super- 

 sylviau almost enters the Sylvian fissure. 



In his description of the brain of the polar bear, Ursus maritimus, Turner' says: 



On opeaiug up the Sylvian fissure I found to my surprise that a definite arched convolution was 

 completely concealed withiu it. It was separated from the convolution which bounded the Sylvian 

 fissure by a deep fissure, which was also concealed. Its anterior limb, not quite so bulky as the posterior, 

 was continued into the supraorbital area immediately external to the rhinal fissure and to the outer 

 root of the olfactory peduncle. Its posterior limb reached the postrhiual fissure and the lobus hijipo- 

 campi. "I could not but think that we had here, more completely than either in the walrus or seals, a 

 sinking into the Sylvian fissure of the convolution which ought to have bounded it, so that both the 

 Sylvian convolution, properly so called, and the suprasylvian fissure were concealed within it. If 

 this be a proper explanation of the arrangement, then the three convolutions on the cranial aspect 

 would be sagittal, mediolateral, and suprasylvian, while the two complete curved fissures between 

 them would be the mediolateral and lateral. 



' Loc. cit. 



