TRITUBERCULV AND SEXTUBERCULV 5c 



lacks tiuality as a convincing proof of tlie tritulicrcular tooth as 

 a primitive Ungulate tooth. 



Professor Osborji has ingeniously utilised certain deviations 

 from the norm.d 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 nfituraUy he in 

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

 molar of Thijlacinv.s " mn\ 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 Zeugloclon. Finally, 

 the modern Toothed Wliales have retrograded into " haplodonty." 



Embryological evidence has also been called in, and with 

 sonre success, to contribute towards the proof of the tritubercular 

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

 Pig, and some other L'ngulates, there is first of all a single 

 liillock 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 iirst, as it obviously ought to do phjdogenetically. 

 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 ^'entricle divides before the 

 auricle ; and of course, phylogeneticall}', 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 

 mmiber 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 becTi 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 

 Mammaha, has been referred to. Sefior Ameghino dwells upon 

 the sextubercular condition of man)' primitive mammals even 

 lielonging to the Eutheria. In a recent comnmnication^ he attempts 

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



' "On the Primitive Type of the Plexodont Molars of Mammals,'' Proe. Zool. 

 Soc. 1899, p. 555. 



