1S92.J MILK-DENTITION OF PROCAVIA C4PENSIS. 45 



teeth of Bidelphys, he claims to have found that, besides the succes- 

 siotial tooth to the 4th premolar, there are present indications of the 

 enamel organs of the successional teeth in connexion with all those 

 which remain ; showing that the adult dentition of the Marsupials, 

 with the exception of the 4th premolar, corresponds with the 1st or 

 milk-dentition of the Placentalia, and not, as Flower and Thomas 

 have held, with the 2nd or permanent one. 



From these and other considerations he argues that the two 

 dentitions among mammals are much more constant than has gene- 

 rally been supposed, and that they are probably of equal value — 

 being developed side by side in the jaw from a common enamel 

 ridge ; and he further points out that while the 1st dentition attains 

 its maximum development in the Marsupials and Cetacea, as we 

 ascend in the mammalian series it diminishes iu importance, so much 

 so that in many animals {e.g. the Seals) it becomes quite rudimentarj', 

 while in others {i. e. Rodents) it possibly disappears altogether. 



Should further enquiry substantiate Kiikenthal's deductions that 

 all mammals develop representatives of both sets of teeth, the 

 advisability of retaining the terms Monophyodoat and Uiphyodont 

 will have to be considered. 



The facts to which I have herein drawn attention (above,pp. 40-42), 

 taken in conjunction with Kiikenthal's assertions just alluded to, 

 show that with regard to teeth present in one dentition only, it is 

 impossible to say for certain, upon mere examination of the dried 

 fckuU, to which set they belong, and even comparative anatomy does 

 not help us much (as in the case of the 1st premolar of Ungulates). 

 We must rely entirely upon the study of development, and must base 

 our determination upon the examination of a series of foetal jaws. 



In view of this I am of opinion that we shall sooner or later find 

 in the rest of the Edentates, the Sirenia, and probably in some 

 Marsupials, that vestigial milk or rudimentary successional teeth, 

 which probably never cut the gum, are almost certain to be present 

 in some form or other — either as calcified structures or simply as 

 enamel organs. 



Should there be found teeth in the foetus showing no signs either 

 of duplication or replacement by vertical successors, there will be 

 good reasons for regarding ihem as belonging to the milk or first 

 dentition, as this is invariably developed first in time. 



From these considerations I should conclude that the vestigial 

 teeth which I have described in Hyra.% (viz., the two posterior upper 

 incisors and the lower canine), together with the upper canine 

 described by Lataste, which has not been seen to be replaced by 

 a successional tooth although sometimes persisting with the per- 

 manent teeth, belong exclusively to the 1st or milk series, which 

 would then read as follows, viz. : — 



1. 2, c. p pm. 2 = 30, 



while the adult dentition would be 



i.ie.^,pm.^,m.?=34(?36). 



