362 NATURAL SCIENCE. May, 



Marsupials branched oft" from the main Mammalian stock antf 

 obviated their difficulties in other ways than by the development of a 

 complete temporary dentition. 



At the time of propounding this lucid and carefully elaborated 

 theory, Mr. Oldfield Thomas pointed out that it would entirely collapse 

 should a more complete replacement be discovered among any of the 

 Marsupials, and he has been the first to acknowledge that this has 

 now been done by Kiikenthal. 



Years ago Professor Huxley stated his belief that the deciduous 

 dentition now found in Marsupials was but a relic of a fuller milk 

 dentition, instancing an analogous loss in the case of the shrew, 

 which has no deciduous teeth, while its near ally, the hedgehog, has 

 a complete replacement. At the time, this belief seemed barely 

 tenable, since in no Marsupial had any trace of a larger number of 

 deciduous teeth been found, while even the one present in most was 

 known to be absent in some forms (Pliascogale apicalis and others). 

 Moreover, fossil Marsupials, such as Tricoiiodoii, show the replace- 

 ment of the posterior premolar, and of that tooth only. This, how- 

 ever, only affords one more instance of the danger of generalising 

 from purely negative results, for Huxley's belief has now been amply 

 justified by Kiikenthal's discoveries, and diphyodontism is proved ta 

 have been the ancestral condition of at least the Didelphydae. 



It is, of course, possible that a complete cycle of development has 

 been undergone, the milk dentition at first appearing as a provisional 

 masticatory apparatus and afterwards persisting while their phylo- 

 genetic predecessors underwent abortion. There is, however, little or 

 no foundation for such a suggestion ; a far simpler explanation is to 

 regard both series as representatives of two consecutive series of the 

 Reptilian teeth. 



In making this supposition, we must, moreover, grant some proba- 

 bility that the true molars were at one time subject to replacement, and 

 this hypothesis also receives confirmation from the recent researches 

 on the Marsupials. 



Kiikenthal appears to have been first led to work at the em- 

 bryonic dentition of the Marsupials from the consideration of the 

 synchronous development of the deciduous posterior premolar and 

 the remaining permanent premolars. This suggested the idea that 

 all the premolar teeth belonged to the same series, and Kiikenthal 

 has succeeded in proving that this is the case by the discovery of 

 rudiments of an almost complete series of successional teeth. 



These embryonic rudiments were found, not only for the incisors 

 and canines, but also for the most anterior premolar, and for the first 

 and second teeth of the true molar series. The only rudiments not found 

 were, therefore, those of the second premolar and of the more posterior 

 molar teeth. Further, these vestigial rudiments of all the suc- 

 cessional teeth are identical in structure with the developing third 

 premolar, which is afterwards cut, replacing its deciduous predecessor. 



