735 



and second milk molar are formed in Anlage ftootli 4 and 5 in tlie 

 scheme of fig. 2) but do not develop. 



In what precedes it has in my opinion been definitely proved 

 that in its constitution the marsnpialian dentition has an essential 

 characteristic in common with the reptilian dentition, namely hama- 

 stichism. For (he dentition of Perameles chiefly consists of endosti- 

 chical teelh (these are the so-called permanent teeth of the Mono- 

 delphians), bnt of the exostichical series the third incisor and first 

 molar functionate permanently, the third milk-molar temporarily 

 together with these endostichical teeth. In the upper jaw with its 

 five incisors the hamastichical character is still moi'e clearly perceived 

 since of these five incisors three belong to the exostichical and two 

 to the endostichical series. 



It is remarkable that in literature one repeatedly comes across 

 remarks, presented in the shape of possibilities or surmises, which 

 fit in perfectly wilh the here briefly framed theory of the marsnpia- 

 lian dentition. If these investigators had not always been influenced 

 by the opinion that the diphyodontism of Mammals and the poly- 

 phyodonlism of Reptiles were identical, only qnanlilatively different 

 phenomena, a more correct conception of the marsnpialian dentition 

 would in my opinion have sooner prevailed. But in this erroneous 

 premise the conceptions of [jrelacteal and [)ost permanent dentitions 

 were rooted and it was these which blocked the road for a right 

 understanding of 'the marsnpialian dentition. So e.g. Rösk says in 

 iiis investigation of the dental evolution of Marsupials : "Es scheint 

 iiiir sehr wahrscheinlich dasz aiich bei Phalangista nicht allein dcr 

 letzte Priimolar, sondern ancli der dritte Incisivus des Oberkiefers 

 aus der zweiten Zahnreihe entsteht." (To me it seems very likely 

 that also with Phalangista not only the last premolar but also the 

 third incisor of the upper jaw is formed from the second dental 

 series). This statement of Rose is similar to that of Dependorf, 

 quoted above, who also, in this case for Perameles, expresses the 

 possibility that the incisors originate from the two dental series. '). The 

 significance of this fact for the identification of the incisors of Mar- 

 supials and Monodelphian Mammals has been explained above. 



A very remarkable discussion as to the manner in which witli 

 the Monodelphians the four milk-molars and their substituting teelh 

 have originated, is found in Woodward's : "Development of the 



1) Id this respect Bensley's statement is remarkable that with Didelpliis the 

 incisors of the lower jaw do not stand in a single row, the second stands more 

 inwardly between the first and the third. (.On the evolution of the Australian Mar- 

 supiatla. Transact. Linn. Soc. London. Vol. IX, p. 187) 



