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Show 44 MR. M. F. WOODWARD ON THE [Jan. 5, secondarily acquired character, developed comparatively late in the evolution of the class. Detailed perusal of his writings shows these conclusions to be drawn to a large extent from the study of the Marsupialia and especially of Thylacinus ; the solitary tooth shed by that animal he regards as the sole representative of the milk or first dentition of its higher allies (Eutheria), believing the rest of its teeth to represent the permanent or successional second dentition of the latter. At the same time (7. p. 2) he has pointed out (and laid great stress on the fact) that the milk-teeth of the Eutheria invariably show a more primitive pattern and shape than those of the permanent or second series which replace them. The latter are often highly specialized ; while the former often (as is especially the case with the Ungulates) agree more or less closely with the permanent teeth of the extinct ancestors of the order. He concludes that when one set of teeth only are present, as in the Cetacea, it is invariably the permanent or 2nd one, the milk or 1st set being either not developed or suppressed. Arguing along the same lines, he considers that when a tooth such as the 1st premolar in many diphyodont mammals is only present in one dentition (even though in many cases it is very early lost), it must belong to the 2nd or permanent series. Thomas (26, 27) has lately accepted Flower's views as to the relation of the two dentitions, and has added largely to our knowledge of the dentition of Marsupials, Edentates, and Monotremes. He shows conclusively that it is invariably the 4th premolar (not the 3rd as Flower thought) which is replaced by a vertical successor in the Marsupials, thus bringing the dentition of Marsupials and Placentals into more complete harmony ; while among the Edentates he has proved the existence of a milk-dentition in Orycteropus (26). More recently Kiikenthal (18), in a preliminary account of some researches on the development of the Cetacean teeth, has sought to show that, exclusive of the Monotremes, there is no such thing as a monophyodont mammal. In all Cetacea (the typical monophyodonts) he finds that rudimentary successional teeth appear in connexion either with the more fully developed functional ones of the toothed-whales or with the functionless tooth-points of the Mysticeti. He argues from this that these supposed typical Monophyodonts are really modified Diphyodonts, and further that their functional or most fully developed teeth belong to the 1st or milk-dentition, and not, as Flower supposed, to the 2nd set. He also advances some reasons for believing that the homodont condition may be arrived at by a splitting up of the primitive complex teeth of an original heterodont type. In the Marsupials , from a careful examination of the developing 1 Since the above was written Kiikenthal has published (Anat. Anz. 1891 p. 658) the details of his work on Bidelphys, giving figures of the rudimentary successional teeth in connexion with the incisors, premolars, and even molars the last being thus shown in the Marsupials to belong to the 1st dentition! These observations I can confirm so far as the incisors and molars of Didelphys are concerned, but in Trichosurus I can find no trace of the teeth successional to the molars. This may be owing to the embryo being too young. |