BRAIN OP THE ZEUOLODONTIDiE. 619 



the corresponding regions of the reptilian brain was emphasized 

 by Elliot Smith (1903), and is shown to distinct advantage in 

 the lateral aspect of the natural endocranial cast (text-fig. 1) here 

 under description. 



This appearance is accentuated by the apparently reptilian 

 " lobus olfactorius " formed by the anterior part of the cerebrum 

 in Sfcromer's specimen. This pai-ticular resemblance is probably 

 superficial. Instead of passing with a gradual expansion into the 

 front end of the hemisphere as the stalk of a pear is attached to 

 the smaller end of the fruit, the olfactory peduncle in the 

 Zeuglodont is attached to the hemisphere on its ventral surface 

 in the characteristically mammalian fashion, in front of the area 

 of which the still prominent tubercula olfactoria form a part 

 (see text-fig. 3). Behind the position of the attachment of the 

 olfactory peduncle there is an obvious depression in the lower 

 and anterior part of the lateral aspect of the cerebral hemispheres, 

 which 1 consider to be a definite Sylvian depression (in the sense 



Text-fiofure 1. 



Mt 



Sag it tat s/nt/s 



Medfsn tentoriaf 

 cfepressfon 



ner^e /^/.»^"«o- -depression 



Gassertan 



qiin(f/fon. 



Lateral viow of natural endocranial cast of Zeuglodon sensitivus, sp. nov. M. 12123. 

 About J nat. size. 



that that term is used in the lower Mammalia). This view is 

 corroborated by the fact that the groove is occupied by the large 

 middle cerebral vessels. Stromer's specimen shows not merely a 

 large portion of the endocranial cast, but also a mould of the 

 whole interior of the brain-case, including the olfactory peduncle 

 from its origin in the olfactory bulb in its insertion into the 

 brain. The length of the olfactory peduncle there is more than 

 double the antero-posterior length of the cerebral hemispheres 

 themselves, while the relative size and shape of the bulbar 

 terminal dilatation is clearly distinguishable. 



I am able to recognise in this cast (M. 12123) that the optic 

 nerves formed the basal angles of Elliot Smith's "prismatic 

 olfactory peduncle" {vide "anterior view" inset to text-fig. 2). 

 Although this involves a considerable reduction of the actual 

 dimensions of the olfactory peduncles in cross-section, as stated 

 by Elliot Smith, it is still obvious that the sense of smell was of 

 significant importance to the Archreoceti. 



