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Show 1899.] PLEXODONT MOLARS OF MAMMALS. 571 deciduous molars in function, and before the first persistent molar has appeared, it can be seen that the last deciduous tooth, having more than the necessary space for its development, is strongly inclined posteriorly, so that it diminishes the space which will have to be occupied by the persistent molars, and increases in the same proportion the space assigned to the replacing molars. This inequality in the development of the molars also explains why the last upper replacing molar of Euminants and Artio-dactyla generally is notably smaller and simpler, not only than the one on its posterior end, but also than tbe one anterior to it. This fourth replacing molar is the last to cut the gum, and must adapt itself to the space left free by the penultimate replacing and the first persistent tooth. Lastly, I have to observe that the sexcuspidate form of tooth, which is represented as the last term of evolution of molars, is very frequent in the oldest Tertiary Mammalia of Europe, and especially in those of the Cernaysian Fauna. To judge from tbe figures of the recent publication by Mr. Matthew on the M a m m a lian Fauna of the Puerco, a great number of Mammals of this epoch also have sexcuspidate inferior molars. Going a step backwards, we can perceive, with the help of Osborn's and Marsh's publications, that almost all the Mammals of the Upper Cretaceous of North America are provided with sexcuspidate or even more complicated (multituberculatej molars. Going another step backwards, the figures published by Marsh enable us to recognize the same type amongst several Jurassic genera, Peralestes, Pera-spalax, Paurodon, Laodon, Dryolestes, &c, which show their posterior molars resembling those of Didelphyidae and of Proteodidelphys. Going still farther backwards, we find the oldest known fragments belonging undoubtedly to a mammal, Microlestes antiquus, with plexodont molars not far removed from those of Proteodidelphys, and with a crown more closely resembling the crown of unworn deciduous molars of certain primitive Ungulates (Prosotherium, Prohegetotherium, &c.) than the molars of the Plagiaulacidae (Plagiaulax, Neoplagiaulax, &c). I do not maintain that the first complicated molars were sexcuspidate, rather than quadri- or quinque-cnspidate. On this point I have sufficiently explained my opiniou iu my memoir " Sur l'Evolution des Dents des Mammiferes." The clear result of all these facts is, that the famous theory of the gradual complication, of triconodonty and trituberculy, is an untenable hypothesis. Nowhere do we meet with the stages leading from haplodonty to plexodonty ; all those which have been mentioned are, on the contrary, as I believe I have demonstrated, but the result of simplification of molars which were formerly more complicated. Plexodonty therefore presents itself as a primitive character, having made its appearance suddenly ; and it is only the theory of fusion which can explain it in a satisfactory manner. |