Discoveries in Mammalian Dentition. 309 



advanced in reduction; and (11.) that Mammals were ])iimi- 

 tively monophyodont and that the milk-dentition was super- 

 added as a secondary development, the development being 

 njiturally most advanced in the highest orders. 



The latter view was adopted and carried out in great 

 detail by myself" *, and therefore now that Dr. Kiikenthal's 

 discoveries have shed a new light on the subject I am imjjelled 

 to express the revised opinion that they have induced me to 

 form. 



The second of the two theories referred to had as its primary- 

 basis the nearly complete monophyodontism of the Marsu- 

 pials, and the moment these were ])roved to have been ever 

 more largely diphyculont than they are at ])resent the whole 

 case would fall to the ground. And such proof seems now 

 to have been found by Dr. Kiikenthal in the nearly complete 

 set of rudimentary successional teeth discovered by him in 

 embryos of Didelphys ; which can hardly be interpreted 

 otherwise than he has done, namely as rudiments of a 

 previously i'unctional second set of teeth. 



Such being the case I am now for my own part prepared 

 to admit that Mammals must have been originally diphyodont 

 and that their regular diphyodontism uas probably in direct 

 succession to the irregular polyphyodontism of their Reptilian 

 ancestors, or may even have existed in what were in other 

 respects members of the latter class. 



At the same time it is evident that on this view many of 

 the known facts seem to become more instead of less difficult 

 of interpretation. Thus the fact that Triconodon^ one of the 

 earliest known Mammalia, changed a single tooth only f, 

 and that the very one which changes in the modern Marsu- 

 pials, now appears most inexplicable, and is alone almost 

 calculated to stagger belief in primitive diphyodontism. 



This problem, however, may be left for time to unravel, 

 but its existence is sufficient to excuse those who, before 

 these latest discoveries were made, could not bring them- 

 selves to believe in that view of the ancestral history of 

 Mammalian teeth. 



The same fact, combined with the presence of four un- 

 doubted premolars (of whichever " series ") in so many of 

 the earliest Marsupials, renders it also difficult, if not impos- 



* Phil. Trans. 1887, p. 44.3. 



t The specimen of Triconodon {Triacanthodon) figured in my paper 

 has, by the kind permission of Dr. "Woodward, been carefully developed 

 beneath all the cheek-teeth, and reexamined by the light of 'Dr. Kiiken- 

 thal's discoveries. No other successional teeth, however, besides that 

 below p.'' are present in the jaw. 



