Fish, Brain of the Fur Seal. 85 



tory peduncle. Its posterior limb reached the postrhinal fissure 

 and the lobiis Jiippocampi. I could not but think that we had 

 here, more completely than either in the walrus or seals, a sink- 

 ing 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 saggital, me- 

 diolateral, and suprasylvian ; whilst the two complete curved fis- 

 sures between them would be the mediolateral and lateral." 



The question quite naturally arises if the fissure concealed 

 in the Sylvian may not be the equivalent of the anterior-postica 

 of Krueg and the two remaining visible on the cranial surface, 

 the supersylvian and lateral. 



The medilateral of other authors does not attain the size 

 nor continued length in the frontal direction as ascribed to the 

 mediolateral by Turner. And furthermore there is in some 

 forms, as in the seals, a well defined medilateral in addition to 

 the two principal fissures. 



In a specimen of Ursiis americamis, I had the good fortune 

 to discover a stage one step beyond that described by Professor 

 Turner. On opening the Sylvian fissure I found in its caudal 

 wall a completely submerged fissure, with a remnant of the Syl- 

 vian gyre which might possibly be mistaken for the insula. A 

 true insula, although small, is present. This submerged fissure 

 I take to be the disappearing vestige of the ectosylvian (Owen) 

 or anterior-postica (Krueg). A study of foetal bear brains with 

 reference to the distinct appearance of the first circumsylvian 

 arch (anterior-postica) would be most important in this condition. 



It would seem then that the condition thus described in the 

 polar bear and American bear would represent the method of 

 disappearance, rather than the appearance, of the first circum- 

 sylvian arch and prepare us for the conditions that we find in 

 the sea lion (Zalophus) and the seals {Phoca and Callorhiniis). 



In the sea lion the conditions regarding the frontal portion 

 of the Sylvian gyre are intermediate between the bears and 

 seals. The presupersyluian fissure approaches very closely to 



