286 Dr. W. Kiikeiitlial on the 



tlius laying the foundation of the theory that the milk-den- 

 tition has been secondarily acquired by the Mammalia, and 

 occurs in ^larsupials only in a single case (the third premolar). 

 Although Flower's deduction was by no means generally 

 accepted, the conception of the ]\larsupial dentition as 

 belonging to the permanent or, to speak more correctly, the 

 second series of teeth, was universally adopted. Thus it is 

 supposed bv Winge*, who otherwise in opposition to Flower 

 regards tooth-change as an old arrangement, inlierited from 

 the lower Vertebrates, that the mi Ik- dentition in the Marsu- 

 pials has been lost, with the exception of one milk-tooth, the 

 precursor of the third premolar. A higher grade wou'd 

 therefore have to be assigned to the dentition of Marsupials 

 than to that of the majority of ^Mammals. " But if, con- 

 trary to all jjrobability, it should a})j)ear that the Marsujiial 

 teeth in question have never had precursors in the course of 

 either ontogeny or phylogeny, they would correspond to the 

 milk-teeth in other Mammals ; but they would be milk-teeth 

 developed to such an extent that in respect of their form &c. 

 they would have to be compared with the teeth of the second 

 series in other ^Mammals." 



Our knowledge of the dentition of Marsupials received a 

 further and very material advance in consequence of Oldtield 

 Thomas's paper f, in which the homologies of the various 

 teeth are determined and the typical Marsupial dentition 

 stated as consisting of 5 incisors, 1 canine, 4 premolars, and 

 4 molars. Reduction set in, and gave rise to the dentition of 

 the various ^larsupials; that of Didelphys arose in conse- 

 quence of the loss of the second premolar. The third pre- 

 molar, which is pi-ovided with a precursor, should therefore 

 really be termed the fourth premolar. Thomas follows 

 Flower in regarding the milk-dentition as having been secon- 

 darily acquired within the ^lammalian class, and consistently 

 follows out tills idea. He himself points out that, besides 

 other things, the possible discovery of the rudiments of a 

 successor in the case of Marsupial teeth which exhibit no 

 tooth-change would be fatal to his theory. 



It was this consideration which guided me in my own invos- 

 tigatlons. If in the eoui"se of development rudimentsof second 



* Ilerliif Wiiijrt', " Om Pattedyrenes Taudskifte, isaer lued Ilenj^yn til 

 Taendernes Former,' VideusJi. Meddel. fra den nuturh. Foi-eu. in Kjoben- 

 havn, 1882, p. 52. 



t Oldtield Thomas. '' On the Homologies and Succession of the Teeth 

 in the Dasyurid;v, with an Attempt to trace the History of the Evolution 

 of Mammalian Teeffi in general,'' Philosophical Transactions of the Royal 

 .'Society (l,ivid<in, 1>^87). 



