ii TRITUBERCULY AND SEXTUBERCULY 59 



lacks finality as a convincing proof of the tritubercular tooth as 

 a primitive Ungulate tooth. 



Professor Osborn has ingeniously utilised certain deviations 

 from the normal type of tooth structure (for the group) in favour 

 of his strongly -urged opinions. If the stages of development 

 have been as he suggests, a retrogression would naturally be in 

 the inverse order ; thus the " apparently ' triconodont ' lower 

 molar of Thylatinua " may be interpreted as a retrogression from 

 a tritubercular tooth. In the same way may be explained the 

 triconodont teeth of Seals and of the Cetacean Zeuglodon. Finally, 

 the modern Toothed Whales have retrograded into " haplodonty." 



Embryological evidence has also been called in, and with 

 some success, to contribute towards the proof of the tritubercular 

 theory of teeth. Taeker has shown that in the Horse and the 

 Pig, and some other Ungulates, there is first of all a single 

 hillock or cusp, and that later the additional cones arise separately. 

 An apparent stumbling-block raised by these investigations is 

 that it is not always the protocone or its equivalent in the upper 

 jaw which arises first, as it obviously ought to do phylogenetically. 

 This, however, is not a final argument in either direction. We 

 know from plenty of examples that ontogenetic processes some- 

 times do not correspond in their order with phylogenetic changes. 

 Thus in the mammalian heart the ventricle divides before the 

 auricle ; and of course, phylogenetically, the reverse ought to 

 occur, since a divided auricle precedes a divided ventricle. This 

 method of development has, moreover, been interpreted otherwise. 

 It has been held to signify that the complex teeth of mammals 

 are indeed derived from simple cones but by the fusion of a 

 number of those cones. 



On the other hand there are the claims of the multituber- 

 cular theory of the origin of mammalian teeth to be considered. 

 The palaeontological evidence has been already, to some extent, 

 utilised. The occurrence of such teeth among the possible fore- 

 runners of mammals, and in some of the most primitive types of 

 Mammalia, has been referred to. Senor Ameghino dwells upon 

 the sextubercular condition of many primitive mammals even 

 belonging to the Eutheria. In a recent communication 1 he attempts 

 to identify six tubercles in the molars of types belonging to a 



1 " On the Primitive Type of the Plexodont Molars of Mammals," Proc. Zool. 

 Soc. 1899, p. 555. 



