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Show 556 SR. F. AMEGHINO ox T H E [May 2, several simple teeth. It is this latter theory which I have been upholding for the last 15 years. In a memoir published about three years ago *, I showed that the tritubercular theory, contrary to what has been asserted, does not agree with the facts furnished by either the embryology, palaeontology, or general morphology of the mammalian dentition. I observed, moreover, that triconodonty and trituberculy, far from being stages leading to the more complicated forms of teeth, are, on the contrary, the result of the reduction of the latter. It was not until after the publication of m y paper that I learned that Dr. Forsyth Major had expressed views similar to mine in the Proceedings of this Society. As a complement to m y preceding work, I shall now endeavour to determine the most primitive type at present recognizable in the crown of tbe lower plexodont molars of Mammalia, In a subsequent paper I shall deal with the upper molars. Firstly a few words on the terms used by me. I recognize in the dentition:- (1) The deciduous molars (milk-teeth) and the persistent molars (true molars), representing together the first series, which is the oldest from an embryological as well as from a palseontological point of view. (2) The replacing molars (premolars), representing the second series, which is of more recent date and always remains incomplete. I assign to the teeth behind the canines the progressive numbers 1 to 7, since they are perfectly homologous in the Placentals and Marsupials, the only difference being that some teeth may belong to the first series in certain genera (e. g. the fourth of Marsupials) and to the second series in others (e.g. the fourth of Placentals). Each of the lower complicated molars exhibits two lobes, an anterior and a posterior, and six cusps or denticles, three for each lobe. According to the authors of the tritubercular theory, these cusps have made their appearance gradually in successive geological periods, and they assign to each cusp a different name. These names have different suffixes in the molars of the two jaws ; furthermore, there are different names for the same cusps in the premolars, for the lobes according to their form, for the colon-nettes (styles) and crests (lophs), &c.; constituting altogether such a complicated terminology, that it remains absolutely unintelligible for all who have not specially studied the argument, and discourages many persons who wish to become initiated in the study of palaeontology. Besides, these names correspond with conceptions which are often uncertain and sometimes preconceived. I shall only make use of the old and vulgar names designating the different parls according to their position. Everv complete plexodont molar has an anterior and a posterior lohe, each of them carrying three cusps. The three cusps of the anterior lobe are the median-anterior, the antero-external, and the antero-internal; the 1 F. Ameghino, " Snr Involution des Dents des Manimiteres," in Eol Acad Nac. de Ciena, t. xiv. pp. 3S1-517 (1896). |