NERVOUS SYSTEM. VERTEBRATA. 289 



There is a great oblique marginal sulcus of Owen ("supra- 

 splenial " of other authors) parallel to the intercalary sulcus 

 (fig. 163) ; it crosses on to the dorsal surface, where it 

 pursues a considerable course (fig. 162). 



The cerebral hemispheres overlap the cerebellum to a 

 much greater extent in this brain than is* the case in the 

 Bear. 



In structure the cerebellum, like that of the Bears, pre- 

 sents the usual Carnivore features. 0. C. 1325 v e 



J. Murie, Trans. Zool. Soc., vol. viii. 1874, p. 517. 



D. 371. A cast o the cranial cavity of a Sea-Bear (Otaria jubatd) . 



D. 372. The brain of a Walrus (Odobcenus [Trichechus] rosmarus). 



The cerebral hemispheres are short and very broad, 

 especially in the postsylvian region. They overlap the 

 cerebellum to a much smaller extent than in Otaria ; and 

 the great caudo-dorsal extension of the hemispheres, which 

 is chiefly responsible for this in the Ct Sea-Bear," is not 

 developed to nearly the same degree in the Walrus. On 

 the other hand, the lateral caudo-ventral region of the 

 hemisphere is widely extended laterally, so as to partly 

 overlap the cerebellum. 



The elongated olfactory peduncles are arranged as in 

 Otaria and all the Seals ; but, as the cast of the cranial 

 cavity shows, the olfactory bulbs are relatively very small. 

 The broad anterior perforated spot, the flattened remains of 

 the olfactory tubercle, and the pyriform lobe resemble the 

 corresponding parts in Otaria. 



The sulci are described in the account o the next 

 specimen. 0. C. 1325 x. 



W.Turner, 'Challenger' Reports, vol. xxvi. 1887, p. 102. 



D. 373. The right cerebral hemisphere of a Walrus (Odobcenus 

 \_TricJiechus] rosmarus) (figs. 164, 165, 166, & 167). 



The sulci of this hemisphere, in spite of the difference of 

 its shape and consequently of the mechanical factors during 

 growth, conform essentially to the same plan as those of 

 Otaria. This leads us to an interpretation of the sulci 

 which is utterly at variance with the views propounded by 



VOL. II. U 



