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Show 418 DR. C. I. FORSYTH MAJOR ON DENTAL [Mar. 15, approximately of the same size; the premolar, however, is slightly larger; a further feature being that, having come in place after m. 1 and m. 2, it is less worn than these two. By the criteria just mentioned, it is possible to determine the anterior tooth in our specimen as the unique premolar, the three teeth following as the three true molars. So that it is the fifth tooth, the last in the series, which is supernumerary. It is distinguished from all the others by its much smaller size, and by reproducing only part of the pattern common to the anterior teeth. Length of the four anterior molars = 35 millim., length of the supernumerary = 4 millim. W e have been told more than once that there are no individual homologies in Mammalian teeth ; the reason, which apparently is considered as the most weighty, being that " on the analogy of what may be seen in the case of Meristic Series having a wholly indefinite number of members, it is likely that the attempt thus to attribute individuality to members of series having normally a definite number of members should not be made." * The " definite number" is just what makes all the difference ; now that order, differentiation, division of labour have been established, as the gradual outcome of what before had been chaos, at least for our short-sighted eyes, we can begin to speak of individual homologies, while before we could speak only of collective homologies. And it is precisely the variations that help us towards making out the individual homologies : witness E. Rosenberg's researches on the variations occurring among the permanent incisors of man f. The old truth that there is a common bond between all the teeth of one specimen, does not invalidate their individuality. They are all brothers, being children of one mother, the dental lamina ; some slight peculiarity in the enamel pattern, by which different species may be distinguished from each other, is often enough common to all or to several of the cheek-teeth of one specimen. In this connection an interesting fact, pointed out by Bateson, is worth mentioning. When a supernumerary tooth is added to the posterior end of the series, the normally ultimate tooth, which has become the penultimate, is not unfrequently abnormally enlarged. This circumstance is considered by the writer to favour his view of the non-existence of individuality, for " the new member of the series seems, as it were, to have been reckoned for before the division of the series into parts." % To me the obvious explanation appears to be, that by the increased activity of the dental lamina, which not unfrequently takes place at the end of the teeth-series, and results in the production of a supernumerary tooth, the normally last tooth has profited as well • so that, as in so many other cases, the variation, the exception, confirms the rule. * W. Bateson, ' Materials for the Study of Variation,' p. 273 (1894) t Morph. Jahrb. vol. xxii. pp. 264-338 (1895). % Proc. Zool. Soc. London, 1892, p. 111. |