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Show 582 MR. M. F. WOODWARD ON [May 5, more constant in connection with the 1st molars, yet were also found in the region of the 2nd molars in several genera. The lingual continuation of the dental lamina was found in all six genera examined, whereas the labial growth was more irregular and only observed in three forms; this latter growth was most conspicuous in Erinaceus, where it was constant from the beginning of ra. 1 to the end of m- 2. A great deal of stress has been laid upon the presence of these structures, especially that of the lingual one, its presence having been said to prove that the molar teeth belonged to the milk-dentition. There is no doubt that if we simply com pare such a section as fig. 26 (Plate X X V L ) , representing the molar tooth of Centetes, with a developing milk-tooth which is known to have a successor we should certainly conclude that the lingual growth of the dental lamina in the two cases was the same structure ; and as it can in one case be shown to give rise to the enamel-organ of a replacing tooth, we might apparently be justified in concluding that in the case of the molar it represented a reduced enamel-germ of a permanent tooth, and that the molar tooth belonged in consequence to the milk-dentition. But it is now well known that we have in the Mammalia traces of three or four sets of teeth; and as it is highly probable that the Mammalia are derived from polyphyodont ancestors, it is possible that there might at any time appear traces of a polyphyodont dentition. It appears, then, to me that presence of a lingual continuation of the dental lamina does not necessarily imply that the labial tooth belongs to the milk-series; it might equally well belong to the permanent or to the post-permanent series, all traces of the earlier labial sets being lost, the lingual growth being not merely the enamel-germ of a successor, but the free end of the undifferentiated dental lamina, which may go on growing and producing fresh sets of teeth, as it does in the polyphyodont reptile, where it is the " anlage " of numerous enamel-organs. Compare for a moment these two diagrams (p. 583): fig. 1 representing a section of the dental lamina of a reptile with a practically unlimited succession, while fig. 2 represents the milk-tooth of a mammal with a lingual development of the dental lamina, which is here known to give rise to a permanent tooth ; we should not in this case be justified in concluding that " m " in fig. 2 was the homologue of 3 in fig. 1, merely because of the presence of this similar development of the dental lamina on its lingual side; we must either conclude that m is the homologue of 1 and the permanent tooth of 2, or perhaps 1 or both 1 and 2 have been completely suppressed, and therefore m is the homologue of 2 or 3 as the case may be. In fact we must start with the dental lamina from the gum, looking most carefully for labial rudiments, so as to be perfectly sure that none of the.earlier sets of teeth have disappeared, before we can homologize the functional teeth, and we must naturally expect to find a lingual growth of the dental lamina constantly present, whether we are dealing with the 1st, 2nd, or 3rd sets, there being no reason to believe that there is an ultimate |