922 MB. R. LTDBKKER OK THE DENT AX FOHMtTIiA OF [NoV. 14 



as this part of the skull is concerned, in deriving the Whalebone 

 from the Toothed Whales. But whether such is the true phylo- 

 geny may be left an open question ; and I may add that, for 

 several reasons, 1 do not propose on this occasion to discuss the 

 geological age of the deposits from which Prosqualodon was 

 obtained. 



6. The Dental Formula of the Marsupial and Placental 

 Carnivora. By R. Lydekker. 



(Plate LXII.) 

 [Received October 21, 1899.] 



Since the views expressed in the ' Study of Mammals ' ^ with 

 regard to the dental succession in the Mammalia generally, and 

 the homology of the individual teeth of the cheek-series of the 

 Marsupials with those of the Placentals, are out of harmony with 

 the results of recent investigations, I ^think the time is ripe for a 

 statement that I, as the surviving author of that work, no longer 

 hold them. And I do this the more readily because it appears to 

 me that some emendations in regard to the names employed for 

 certain of the teeth of the cheek-series are urgently required. 



I may commence by the statement that I fully accept the view 

 that the milk-teeth ^:>?«w the so-called true molars constitute the 

 first, or original series, and that the premolars form the second 

 series ; this being precisely the opposite of the view taken in the 

 work referred to -. Apart from other considerations, 1 regard the 

 fact that the last tooth of the milk-molar series (as well as some- 

 times the tooth in advance of it) is always similar in structure to 

 the true molars as a very strong argument in favour of this view. 

 And I likewise accept the view that the whole of the teeth of 

 modern Marsupials, with the exception of the single replacing pair 

 in each jaw, belong to the fii'st series. 



This being so, I come, without further preliminaries, to the 

 consideration of the special subject of the present communication ; 

 that is to say, the serial homology of the individual cheek-teeth in 

 the Marsupial and Placental Carnivora, and the dental formula 

 that will best express this homology. It will simplify matters to 

 confine our attention in the main to the teeth of the lower jaw, 

 as what holds good for these will be likewise applicable iu the case 

 of those of the upper jaw. 



To go no further back than the publication of his ' Odonto- 

 graphy,' we find Sir E. Owen in that w^ork ' giving the lower 

 dental formula of Canis, which may be regarded as typical for the 



' Flower and Lydekker, 1891. 



- I do not propose to take into consideration the evidence in favour of the 

 occasional presence of an aborted successional series to the true molars. 

 » Page 475. 



