736 



Teetli of the Maoropodidae" (Proc. Zool. Soc. 1893). The audior 

 points out that willi Amphilestes "tliere are J 2 or 13 cheek-teeth 

 present, and no evidence of' tlie presence of two sets of teeth. May 

 not the five posterior ones", the author continues, "represent the 

 five molars (Bettongia) while the first 8 might be supposed to give 

 rise to the 8 premolars, (4 milk and 4 permanent) and bj the 

 retardation of eacii alternate one the condition in the Placentalia 

 might be brought about, the 2"', 4"', B''' and 8"' being retarded 

 and displaced to form a second or replacing set, while the 1«', 'S^, 

 5''' and 7''' develop early and are replaced by the former" (loc. cit. 

 p. 470). So here the toolh-change is explained as a possible shifting 

 into two rows of a larger number of teeth in the most primitive 

 mammals. 



If WooDWAKD had known that two-rowedness is an essential 

 characteristic of the dentition of Reptiles and certainly also of the 

 Marsupials, he would ha\e explained the relation between the 

 dentition of Amphilestes and the Marsupials with four premolars in 

 the opposite direction and then in my opinion correctly. It seems 

 to me that with Amphilestes hamasthichism is still fidly expressed, 

 i. e. the four teeth of the endostichical row push themselves between 

 the four teeth of the exostichical row as with the Reptiles. In the 

 now living Marsupials this hamastichism has for the greater part 

 been lost by a number of exostichical teeth becoming rudimentary, 

 with the Placentalia it has been entirely lost exactly on account of 

 the more complete development of the exostichical teeth, by which 

 the endostichical ones were retarded and the foundation was laid 

 for a system of tooth-change, in which the exostichical teeth were 

 replaced by the endostichical. 



Now this process deviates in its essential points entirely from 

 that of the Reptiles. But for a correct insight into these relations 

 the knowledge of the structural principle of the distichism of the 

 reptilian dentition was required. And in this communication it has 

 been proved in principle that this distichism leads with Reptiles 

 to a hamastichically built dentition, with the Monodelphian Mammals 

 to a chorisstichically built one, while the Marsupials form a transi- 

 tion between them, as their dentition is still partly hamastichical. 

 one element being with many forms replaced in the same manner 

 as with the Monodelphian mammals, so that also the phenomenon 

 of cliorisstichism is already present in principle. But it should be 

 clearly pointed out thai the question to what extent the marsu|)ialian 

 dentition is still hamastichical will have to be solved for each form 

 separately. 



