618 prof. u, a. dart on tub 



3. The Two Enbocranial Oasts unaccompanied by 

 Osseous Remains. 



Unfortunately there are no bony remains associated with the 

 natural cast (M. 12123) spoken of at the outset. This cast, 

 altliongli belonging to some Zeuglorlont, is so entirely diflCTent 

 from any one of the phyletic series that it certainly comes from a 

 species of Zeuglodont not known in the British Museum collection. 

 Lack of information prevents our associating it with any other 

 Zeiiglodont hitherto described. This cast presents chai-acters 

 which indicate its affinity with the natural cast (M. 120G6) 

 described by Elliot Smith (1903) — which also was unaccompanied 

 by any osseous remains — i-ather than with any member of this 

 so-called phyletic series. This natural cast does not come from 

 another member of the same species as that described by Elliot 

 Smith, nor is it an endocranial cast of Zeuglodon osiris. Ooin- 

 parison with a duplicate (M. 12066) of the natural cast described 

 by Elliot Smith and with Zeuglodon osiris (M. 10228) demon- 

 strates a wider degree of separation from both of these than could 

 bo accounted for on a specific differentiation alone ; we may be 

 dealing here with different genera. 



In the case of these casts (M. 12123 and M. 12066) the absence 

 of knowledge concerning the skeleton and the exact horizon from 

 which they have come makes it an invidious matter to establish 

 new species. At the same time it is necessary, in order to avoid 

 confusion in description, to associate some name with each of 

 these casts since they represent at least different species, if not 

 genera. Since we owe to Professor Elliot Smith our first detailed 

 account of the brain in the Archaioceti, and especially the recog- 

 nition that '• the dilierences" (i. e., between the endocranial cast 

 of Zeuglodon osiris and tlie natural cast (M. 12066) in his hands) 

 "are sufficiently pronounced to indicate a generic distinction 

 between the two specimens " ; I propose to term the form from 

 which this type of cast is derived Zeuglodon elliotsmithii, sp. nov. 

 The second natural cast (M. 12123) — the one which Professor 

 Elliot Smith has given to me for description — I will call 

 Zeuglodon sensitivus, sp. nov., since it was in this cast that a 

 marked hypertrophy of the trigeminal apparatus was first 

 recognised. 



3, a. Zeuglodon SENSITIVUS, sp. nov. (M. 12123.) 



The most complete endoci^anial cast of a Zeuglodont yet 

 discovered is that represented by Stromer (1908) in situ, in the 

 skul] of what he has there called Zeuglodon osiris. It is not 

 the Zeiiglodon osiris of his earlier (1903) paper, but more nearly 

 resembles the Prozeuylodon atrox Andrews or is intermediate 

 between Prozeuglodon atrox and the Zeuglodon intermeditts of 

 this paper. 



The peculiar resemblance of the general configuration of the 

 fore-brain and olfactory peduncles in the Zeuglodont brain to 



