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Show 1892.] MILK-DENTITION OF PROCAVIA CAPENSIS. 45 teeth of Didelphys, he claims to have found that, besides the successional 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 generally 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 in importance, so much so that in many animals {e. g. the Seals) it becomes quite rudimentary, 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 Monophyodont and Diphyodont 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 skull, to which set they belong, and even comparative anatomy does not help us much (as in the case of the 1st premolar of Ungulates). W e 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 them 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 Hyrax (viz., the two posterior upper incisors and the lower canine), togetber with the upper canine described by Lataste, which has not been seen to be replaced by a successional tooth although sometimes persisting with the permanent teeth, belong exclusively to the 1st or milk series, which would then read as follows, viz.:- • 3 1 4 on i.-, c. v pm. ~ = 30, while the adult dentition would be J.*, c. ^,pm.J,m.J=34(?36). |
