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Show 470 MR. M. F. WOODWARD ON MAMMALIAN DENTITION. [May 2, not this retardation of the 4th premolar as seen in the Macropodidae be the first step in the formation of the permanent set1, which may afterwards take on a secondary connection with the teeth of the 1st dentition? In Amphilestes there are 12 or 13 cheek-teeth present, and no evidence of the presence of two sets of teeth. May not the five posterior ones represent the five molars (Bettongia), while the first 8 might be supposed to give rise to the 8 premolars (4 milk and 4 permanent), and by the retardation of each alternate one the condition in the Placentalia might be brought about, the 2nd, 4th, 6th, and 8th being retarded and displaced to form a second or replacing set, whilst the 1st, 3rd, 5th, and 7th develop early and are replaced by the former? These teeth, which were originally distinct, may have acquired a secondary connection with the tooth in front, as seems to have been the case with the 3rd premolar of the Kangaroos and the one replacing tooth, this latter in those forms displacing both the 3rd and so-called 4th premolar. In others it, however, only displaces the so-called 4th premolar, owing to the latter having, through its enlargement, acquired a connection with the replacing tooth, as in Bidelphys; or, owing to the reduction in size of the 4th premolar, as in Perameles and Thylacinus, the supposed replacing tooth is able to cut the gum iu its more normal position and displaces the reduced tooth behind. If these various and often minute cord-like downgrowths of the dental lamina are to be interpreted as representing rudiments of teeth, as seems probable from comparison with the known rudiments of the 1st or 2nd dentition in other mammals, then we find that in the Kangaroos the incisor teeth all belong to the 1st dentition, that the relations of the canine are uncertain, that tbe premolars probably belong to the 1st dentition, whereas the molars, or at any rate the 1st, belong to the 2nd dentition. This last statement is a reversion to older ideas as to th& relation of these teeth, held by all odontologists prior to the appearance of Kiikenthal's paper, wherein he formulated the theory that the molars belonged to the 1st dentition. As I have already pointed out, he has retracted part of his statements on this point, and I have been unable to confirm his views as to the 1st molar iu Bidelphys, while in the Macropodidae I have apparently found exactly the reverse condition". I should suggest by way of explanation as to the presence of the permanent molars in a dentition which w:as otherwise entirely composed of milk-teeth, that owing to the shortness of the jaws the molars were formed very late, and owing to the inability to find room for two sets of what are naturally large teeth, the 1st or milk dentition, as the least important, became suppressed, and is only seen as a slight rudiment attached to the least modified molar, 1 Similar to the condition seen in the Monitor amongst Reptiles. 2 For the present I leave out of consideration Leches (6) account of the condition and homology of the molars of Erinaeeus, as I have not yet finished my observations on the molars of the Placentalia. |