372 Dr. W. Kiikenthal on the Origin and 



mammals also a fusion of the rudiments of both dentitions 

 occurs in the formation of the true molars *. 



If the multituberculate molars have arisen in this fashion, 

 it follows that their number must be very small, since each 

 tooth corresponds to a whole series of simple reptilian teeth. 

 As a matter of fact we find in each half of the jaw onlj one 

 or two molars, while the number of the similarly constructed 

 premolars is at the most four, but usually less. It is difficult 

 to understand how the process of fusion has taken place, 

 since the shortening of the long jaws of the reptiles to the 

 short ones of the mammals is not of itself a sufficient explana- 

 tion ; nevertheless the fusion of teeth in the vertebrates is a 

 fact, and consequently my view is in no way opposed to 

 processes of tooth- formation in lower Vertebrata. 



If the mammalian molars have really arisen as I have 

 suggested, the hypothesis which is at present generally 

 accepted, and has been especially developed by Cope and 

 Osborn, is consequently invalidated up to a certain point. 

 Starting from the simple conical reptilian tooth, such as, 

 according to these authors, has been preserved in the dolphinf, 

 the development of the mammalian molars is supposed to 

 have taken place by the outgrowth of a small cusp in front 

 and behind. The difficulty of conceiving the mechanical 

 process of such an outgrowth has already been touched upon 

 by Fleischmann J, since Cope's attempt to explain the deve- 

 lopment of these cusps, as being due to the increased supply 

 of formative material, is an absolute failure. But the diffi- 

 culty is abolished if the triconodont and tritubercular teeth 

 are regarded with me merely as constituting a special division 

 of the multitubercular teeth, and therefore as structures which 



* This view also, which I expressed on the basis of my investigations, 

 is regarded by Thomas (loc. cit. p. 311) as an " extraordinary and, to all 

 appearance, most unhkely theory." Without here entering into further 

 explanations, I will merely refer the reader to p. 231 of Hertwig's ' Lehr- 

 buch der Entwicklungsgeschichte des Menschen und der Saugetiere,' 

 where it is stated : — " In addition to this the enamel organs of the poste- 

 rior or true molars, which are subject to no change, but of which the 

 rudiments are altogether only formed once, are developed at the right 

 and left end of the two epithelial folds." These two epithelial folds are, 

 however, nothing else than the earliest rudiments of the enamel organs 

 of the first and second dentition, which in the case of the premolars 

 remain separate. 



t Thomas is in error in thinking that this view is only shared by 

 Baume ; vide, e. g., Schlosser, " Die Dift'erenzieruug des Saugetiergebisses,'' 

 Biol. Centralbl. 1891, p. 238. 



X Fleischmann, " Die Grundform der Backzahne bei Saugetiereu und 

 die Homologie der einzelnen Hiicker," Sitzungsber. der k. Akad. Berlin, 

 1801. 



