1887.] 



On the Teeth of the Dasyuridce. 



311 



Firstly, as to which, of the three premolars of such genera as 

 Thylacinus and Phascologale have been lost in Dasyurus and Sarco- 

 philus, each with only two — a study of the different members of the 

 genus Phascologale shows that, judging by the great variability in 

 size of the last premolar or pm. 4 of the typical mammalian dentition,* 

 which is sometimes even altogether aborted, it is this tooth that is the 

 one lost in Dasyurus and Sarcophilus, the total loss of the changing 

 tooth naturally accounting for the non-discovery of a tooth- change 

 in these genera. 



Next, since the original number of premolars was clearly four in 

 the Marsupials as well as in the Placentals, it was necessary to find 

 out which of these had disappeared in the ordinary three- toothed 

 genera of the Polyprotodonts, and this has been able to be done by 

 the fortunate discovery of a specimen of Phascologale in which there 

 are four premolars on one side, the additional tooth being inserted 

 between the ordinary first and second premolars. The missing pre- 

 molar is therefore pm. 2 , as shown both by this instance and by the 

 relative positions of the teeth in other Polyprotodonts, the resulting 



premolar formula of Phascologale and Thy lacinus being P.M. ' ^ ' jj ' ^ 



and of Dasyurus and Sarcophilus P.M. ^ .f 



The milk dentition in several of the Dasyuridce is then described, 

 among others that of the Purbeck Mesozoic Marsupial Triacanthodon 

 serrula (Owen), which is proved to have, as had been suggested by 

 Professors Owen and Flower, a milk dentition identical with that of 

 the modern Marsupials. 



An attempt is then made to trace out the history of the evolution 

 of mammalian teeth in general, and as a preliminary it is insisted (1) 

 that the rudimentary tooth-change of the Marsupials is not a remnant 

 of a fuller one, but a low and early stage in the development of com- 

 plete dipbyodontism, a stage out of which the Eutheria have long ago 

 passed ; and (2) that, as maintained by Professor Flower, the milk 

 teeth are the superadded and not the primary set. 



It is then suggested that the process by which a milk tooth was deve- 

 loped consisted of two stages, firstly, a preliminary retardation of the 

 permanent tooth, and secondly, of the development of a temporary 

 tooth in the gap in the tooth-row caused thereby ; the retardation in 

 the first case being useful for packing purposes in a large-toothed 



* Although the homology of this tooth with the pm. 4 of Placentals, first made 

 out by Professor Flower, has been called in question, there can be no doubt that it is 

 entirely correct. 



f This method of writing dental formula? is recommended as showing not only the 

 total number, but the homologies of the teeth, each of which has its own number in 

 the series. 



