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Show 1892.] NUMERICAL VARIATION IN TEETH. Ill than the other, which Mr. Thomas tells me is abnormally small for the species. In the upper jaw of a normal skull there are two small premolars (p1 and p3 of Thomas) and behind these four molars. The molars increase in size from the first to the third, which is by far the largest. Behind the third is the fourth molar, which is much thinner than the others. On comparing the abnormal skull with the normal one it is seen, firstly, that on the left side there are seven teeth behind the canine, while on the right side there are only six such teeth, as usual. On the right side, however, the last molar has not the thin flattened form of the last molar of a normal skull, but is a fair-sized thick tooth. In each lower jaw there are seven back-teeth instead of six. In making a more detailed comparison, the first five teeth on each side are clearly alike, while from its form the seventh on the left side might be thought to represent the normal sixth, and this is the view originally proposed by Mr. Thomas in his ' Catalogue of Marsupialia,' p. 265, note. The difficulty in this view is that it offers no suggestion as to the nature of the sixth tooth on the right side. In the light, however, of what has been observed in other cases of extra molars, it seems likely that on the right side m 4 has been raised from a small tooth to one of fair size, while on the left side the process has gone further, m 4 being still larger and another tooth having been formed behind it. Mr. Thomas, to whom I am greatly indebted for having first shown me this specimen, allows me to say that he is prepared to accept the view here suggested. This phenomenon, of the enlargement of the terminal member of a series when it becomes the penultimate, is not by any means confined to teeth, for the same is true in the case of ribs, digits, & c , and it is possibly a regular property of the Variation of Series of Multiple Parts which are so graduated that the terminal member is the smallest. This fact will be found of great importance in any attempt to conceive the physical process of the formation of Multiple Parts, and, pending a full discussion of this and kindred processes, it may be remarked that such a fact strikingly brings out the truth that the whole Series of Multiple Parts is bound together into one common whole, and that the addition of a member to the series may be correlated with a change in the series itself, and may occur in such a way that the general configuration of the whole series is preserved. In this case the new member of the series seems, as it were, to have been reckoned for before the division of the series into parts. This is, of course, only one way in which numerical Variation may take place; for, as was described in the previous section, additions to the series may be formed by the division of single members of the series, and in this case the configuration and proportions of the rest of the series remain normal. Examples of these two distinct methods of numerical Variation occur among Series of Multiple Parts of many kinds (digits, vertebrae, &c). Re-constitution of Parts of the Series. Some curious instances of what is almost a remodelling of parts |