648 i)U. C. W. ANDREWS ON TllK 



a I'olatiouship piiuiarily not to the cerebelluiu as such, but to the 

 trigeminus. 



So, in addition to the fringing above mentioned, a graphic 

 picture of the central role which the trigeminal has played in the 

 origin of the cevebelluui is afforded by the relation of the anterior 

 and posterior spinocerebellar tracts to the trigeminal nerve 

 (in most Yertebrata) and by the central position of the trigeminus 

 in the pons (in higher Mammalia), 



Considering the nature of the impulses it conveyed, it is not 

 surprising that the trigeminal nerve, which has had such diverse 

 evolutionary possibilities in all vertebrate groups, should have 

 been very intimately conceined in the emergence and evolution 

 of the cerebellum in Vertebrata. Nor is it surprising that we 

 should find illustrated in the Zeuglodontidae a group of primitive 

 mammals taking to a water life at a stage so plastic that the 

 trigeminal apparatus should become sufficiently specialised and 

 hypertrophic to determine, through its moulding of the cere- 

 bellum and the spinal cord, the Avhole course of evolution (or 

 devolution) of its central nervous system. 



9. Note on the Skulls puom aviiioii the Endooranial Casts 

 DicsoniiJEu ]JY Dii. ])aiit wEiiK TAKEN, l^y C. W. Andiiews, 

 J3.Sc., F.ll.S. (British Museum of Natural History.) 



(Published by permission of the Trustees.) 



Unfortunately, from the nature of the case, the skulls belonging 

 to the two natural casts above described are unknown, having 

 been destroyed by weathering, but those from which the remaining 

 casts were made are preserved and are here discussed. Four such 

 skulls are known. Of these, two [M. 8150 (a cast) and M. ] 0228J 

 certainly belong to Zeuglodon osiris Dames, from the Qa,sr-el- 

 Sagha beds (Upper Mokattam = Bartonian), north of Lake l^irket- 

 el-Qurun in the Fayum. One (M. 9266) is the skull of 

 Prozeuglodon atrox Andrews, from the lower part of the Birket- 

 el-Qurun beds in a valley twelve kilometres W.S.W. of the 

 Gar-el-Gehannem, lying to the west of the lake. This last speci- 

 men may be regarded as the paratype of the species and is 

 described and figured in the British Museum Catalogue of the 

 Fossil Vertebrata of the Fayum (1906, p. 243). The fourth skull 

 (M. 1.0173), forming the middle term of tlie series, seems from 

 the nature of the matrix to have been obtained from the Bii-ket- 

 el-Qurun beds at the western end of the lake, from an horizon 

 intermediate between those from which the other species were 

 found. This, however, in the absence of deiinite information 

 from the collector is not certain. This sk\dl appears to belong to 

 a hitherto-undescribed species which has been called Zeuglodon 

 intermedins by Dr. Dart, the chai-acters of which are given 

 below. 



Numerous other Zeuglodont remains have been collected from 

 various horizons in Egypt, and there is some difleience of opinion 



