Development of the Mammalian Phylum. 369 



divergent in form ; tlie Anomodontia possessed only a pair of 

 powerful tusks (similar to those of the walrus) in the upper 

 jaw, or were completely edentulous. 



The Placodontia, whose right to be included in the order of 

 the Theromorpha is, however, not certain, were still more 

 singularly equipped, since they possessed incisors in front, and 

 posteriorly rounded molars in the up|)er jaw and large flat- 

 tened teeth in the mandible, while in addition to these the 

 palate was covered with large flattened teetli. A precisely 

 similar dentition is found, moreover, in fossil fishes — the 

 Pycnodontia — to which these reptiles were at first assigned. 



If we disregard for the moment the two last-mentioned 

 groups, and devote our attention to the Pareiasauria and 

 Theriodontia, we are especially struck by the fact that here 

 we are not confronted, as in the case of other reptiles, with a 

 succession of several dentitions, which are capable of excellent 

 preservation in fossils (compare, for instance, the figure of 

 Diplodocxis longus, Marsh, in Zittel's ' Handbuch der Palaon- 

 tologie,' iii. Bd. p. 716, where no fewer than six consecu- 

 tive successional teeth are developed) ; but that in these 

 animals there takes place only a single succession or none at 

 all — the latter being the case in the most specialized denti- 

 tions. Within the order of the Theromorpha therefore the 

 formation of successional teeth is lost as the individual teeth 

 become more highly specialized. 



We meet with perfectly analogous conditions once more 

 in the mammals *, for in the marsupials also the second 

 dentition is suppressed with the exception of one premolar, 

 although it is present in the form of a rudiment (the dental 

 fold) ; here also the teetli of the first dentition, which alone 

 arrives at developuient, are highly specialized. 



A very material advance in the completion of the dentition 

 is not exhibited until we come to the placental mammals, in 

 which (with a few exceptions, which will be dealt with 

 directly) highly specialized teeth of the second as well as of 

 the first dentition are developed. W^ith this we have attained 

 the highest known stage of dental development. As regards 

 tiie exceptions, the toothed whales and the edentates, 1 have 

 already shown in my address delivered on this occasion last 

 year, that the condition of their dentition is a secondary one, 

 since the primitive specialization of their teeth appeared no 

 longer necessary, in consequence of diminution of their difFe- 



• Vide my papers in the ' Anatomisclier Anzeiger,' 1891, pp. .364 and 

 658 [Ann. k Ma<r. Nat. Hi.st. ser. 6, vol. i.x. pp. 279-21)4], as well as my 

 address delivered on May 30, 1^*91, " Ueber i\vr\ Ursprun<r imd die Ent- 

 Avicklun<r der -SaugetierzaLne," Jenaisclie Zeitscluift, 1892. 



