72i 



and so to arrive at the conclusion that it cannot be right. I propose 

 in what follows to investigate (he mor[)hological significance of the 

 Marsiipialian dentition, starting from quite different viewpoints. 



A few years ago the Proceedings of this Academy contained a 

 paper bj myself on the relation between the mammalian and rep- 

 tilian dentition. In particular the question was dealt with whether 

 the diphyodontism of Mammals might be derived from the polyphy- 

 odontism of Reptiles by diminishing tlie number of tooth-changes. 

 In such a derivation it is tacitly assumed that the tooth-changing 

 process in Reptiles and in IVIauimals are identical phenomena. In 

 the paper mentioned and in later more extensive papers it has been 

 shown that this supposition is not correct. The tooth-changing process 

 of Reptiles and that of Mammals are two phenomena different in 

 principle. Hence we may not derive the diphyodontism of the latter 

 from the polyphyodontism of the former. And in order to prevent 

 confusion that might be caused by the meaning of these words in 

 which the older conception is reflected, it is desirable to drop these 

 terms and to indicate by other terms what is essential in the mam- 

 malian and reptilian dentition. These terms will be given presently. 



In my investigation on the morphological significance of the Mar- 

 supialian dentition I have from the outset started from another point 

 of view than preceding authors. P^or them the question was in what 

 relation the dentition of the Didelphian mammals stands to that of 

 the Monodelphians, what could be found in Marsupials with their 

 absence of a tooth-change of the two dental series of the .Mono- 

 delphians. For me the principal question was; to what extent do 

 we still find in Marsupials during the Anlage of the dentition phe- 

 nomena that are characteristic for the dentition of Reptiles? For a 

 right understanding of the answer to this question a short account 

 must precede of the chief evolutionary phenomena of the reptilian 

 dentition as compared with (hat of Mammals. 



In the mentioned paper it has been shown that the reptilian 

 dentition originates from tooth-germs, evolving in two rows on the 

 feethbaud, one row on the buccal side close below the epithelium 

 of the cavity of the mouth and a second row on the free border 

 of the feethbaud. Both rows consequently lie as an outer and an 

 iimer one with respect to one another, for which reason they are 

 distinguished as Exosfichos and Endosticlios. And since the first 

 Anlage of the dentition is double-rowed it may be indicated as 

 "distichical". 



Another characlei'istic is that the tooth-germs of both rows alter- 

 nate with each other. First the tooth-germs of the exostiehical row 



