TRITURKRCULY AND SEXTU15ERCULV 59 



lacks tiualitv as a convinciiio- proof of the tritul )ercular tooth as 

 a primitive Ungulate tooth. 



l*rnfessor Osborn has ingeniously utilised certain deviations 

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

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

 have been as he suggests, a retrogressicni would naturally he in 

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

 molar of Thijlacuius " 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 Zoifjlodon. Finally, 

 the modern Toothed Whales liave retrograded into " haplodonty." 



Endiryological 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 

 rig, and some other I'ngulates, there is first of all a single 

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

 An apparent stund:)ling-I»lock raised by these investigations is 

 that it is not always tlie protocone or its eijuivalent in the upper 

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

 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 ])hylogenetic changes. 

 Thus in the mannualian 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 

 jiiethod of development has, moreover, l»een interpreted otherwise. 

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

 are indeed deri^'ed from simple cones but by the fusion of a 

 ninnber of those cones. 



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

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

 Tlie palaeontological evidence has been already, to some extent, 

 utilised. The occiuTence of such teeth among tlie possible fore- 

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

 Mammalia, has l»een referred to. Sehor Ameghino dwells upon 

 the sextuijercular condition of many primitive manunals even 

 belonging to tlie Eutheria. In a recent comnmnication^ he attempts 

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



^ "On the Primitive Type of the Plcxodont Molars of Mammals," Proc. Zvol. 

 Soc. 1899, p. 555. 



