DK. C. W. AXDKKWS ON TIIK 



ii unis a small shelf-like ridiie at the anterior end of the tooth and a larger one 

 behind ; in wear this latter becomes connected with the worn surface of the postero- 

 external cusp, and it forms the rudiment of a third ridge. The next molar is similar. 

 The last is likew ise bilophodont, but has also a large tuberculate talon. 



The relatively primitive character of McBritherium is shown by the fact that the 

 skull presents no very striking peculiarities beyond the forward extension of the 

 maxilla" and the great thickness of the cranial bones. In the mandible the 

 massive symphysis is little elongated. In the upper jaw only one tooth (Pm. 1 ) of 

 the full Eutherian dentition is wanting; in the mandible three (I. 3, C. and Pm. 1) 

 have been lost. The incisors are becoming specialised, but the premolars and molars 

 are of a simple type, though they already show a tendency to increasing specialisation 

 along the lines subsequently pursued throughout the group. 



The discovery of an early and comparatively generalised type like Mrvritherium 

 naturally raises the question of the relationship of the Proboscidea to the other 

 mammals, and although at present it is not possible to arrive at any definite 

 conclusion as to the origin of the group, the view put forward by Blainville and 

 others that they may be related to the Sirenia receives some support. It must be 

 said, however, that several writers* have strenuously opposed this opinion. 



The relationship between the two groups must necessarily be a remote one since 

 a Sirenian (Eosiren), very little less specialised than Mariettas or at any rate than 

 Haliiherium, occurs associated with Moeritherium^ so that a common ancestor of the 

 two groups cannot have existed later than the Lower Eocene. Nevertheless, the 

 following characters are common to the two : — 



(1) Placenta non-deciduate and zonary. 



(2) Pectoral mammae. 



(3) Abdominal testes. 



(4) Bifid apex of heart. 



(5) Absence of the condylar foramen, always in the elephants and generally in the 

 Sirenia, where, however, it may be bridged over by a thin strand of bone. 



((J) The bilophodont character of the molars with a tendency to the formation of an 

 additional lobe from the posterior part of the cingulum (talon). The molars of some 

 Sirenia are extremely like the anterior molars of Moeritherium. 



(7) In both groups the molar series moves forward in the jaws throughout life, the 

 anterior worn teeth being shed while fresh ones come into use behind. In the 

 Sirenia this is effected by the continuous addition of teeth similar to the other molars, 

 while in the Proboscidea no fresh teeth are added but the individual molars become 

 longer and more complex through the addition of further transverse crests to their 

 posterior ends. 



* See for instance T. Gill, "On the Affinities of the Sirenians," 'Proc. Acad. Nat. ScL, Philadelphia,' 

 1873, p. 262. 



