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Show 578 MR. M. F. WOODWARD ON [Mayo, addition the entire absence of any labial growth in connection with pm. 1, which one might naturally expect to find if the functional pm. 1 was ppm. 1, and if Bate's specimen was an exceptional one in which dpm. 1 had been retained. One of the greatest difficulties met with in the study of tooth ontogeny is the want of a sure method for the determination to which set a given tooth belongs, for we may be dealing with a retarded member of an early set or an accelerated development of a later series, and, so far as I can judge, the identification can only be made through a study of the comparative morphology and phylogeny of the tooth, and not by its ontogeny alone. That the time of appearance of the enamel-organ does not help us is well seen in the Mole, where the germ of pm. 1 appears after the other milk-teeth and at the same time as pc.; but this latter tooth appears long before the other permanent teeth, so that if we took the time of appearance of these tooth-germs as a criterion we should have to conclude that the deciduous incisors, canines, and three posterior premolars belonged to one set, the 1st premolar and permanent canine to a second set, and the other permanent teeth to a third series, a conclusion which, I think, condemns itself in the mind of all those who have studied this subject. Such a suggestion was put forward many years ago by Wortinan (31), who regarded the four molars of the Placentalia as belonging to four distinct sets of teeth; this view does not appear to have met with any general recognition, it being more natural to suppose that the dental lamina though temporarily fused wdth the germs of the anterior molars yet retains its individuality and grows back with the elongation of the jaw to form fresh teeth belonging to the same series as the more anterior molars. The only doubt arising in m y mind as to whether I am right in referring the first premolar, in the Mole and in all animals where it is only known in one dentition, to the milk-series and so terming it dpm. 1, is due to the appearance seen in Erinaceus; for if in that genus the apparent tooth-vestige which I have mentioned (ante, p 562) as occurring between the two posterior upper premolars really represents a lost premolar, then the anterior premolar of Erinaceus is the true pm. 1; and as further I have shown that the deciduous predecessor of that tooth is a vestigial structure, the functional tooth must be referred to the replacing dentition. Consequently, if the above premises be true, we have here an example of the suppression of dpm. 1 and a persistence of ppm. 1, a conclusion antagonistic to that which I have arrived at concerning this tooth in the Mole, and I could only suppose that the homology of this tooth (pm. 1) varies in different and closely related animals. 1 have thought it only fair to give this possible objection to my view here, but, as I have already mentioned, this supposed vestige of pm. 3 in Erinaceus is very slight and has not been observed by Leche in any of his stages ; so it is highly probable that this structure has no morphological importance, and Leche's identification of the 1st functional premolar in this genus as pm. 2 may be quite |