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Show 924 MR. R. LYDEKKER ON THE DENTAL FORMULA OF [Nov. 14, and that the true homologue of pm. 4 is not developed in the Marsupialia." In this passage, then, the great anatomist recognizes, firstly, that the first true molar of Placentals belongs to the first series of teeth ; and, secondly, that the fourth cheek-tooth of the Marsupials is a persistent last (fourth) milk-molar. And it is merely in order to obtain general recognition for these two important facts that the present paper is chiefly written. The next amendment in the dental homology of the Marsupial and Placental Carnivora was made by Professor A. Gaudry **, in 1878, who, struck by the resemblance between the teeth of the Creodont and Marsupial Carnivora, applied the same formula to both, thus making the lower dentition of Thylacinus i. 3, c. 1, j>. 4, m. 3, or the same as that of Hycenodon and Canis. H e then pointed out that although the Creodonts differed from Thylacinus and its allies by a complete dental replacement, yet the former likewise differed from modern land Carnivora by the circumstances that all their three true lower molars were of a carnassial type, and that they closely resembled the corresponding lower teeth of the Thylacine. No attempt was, however, made to show why the latter animal, in common with its kindred, should have four teeth of this same carnassial type. In 1887 appeared a paper by Mr. O. Thomas 2, iu which the replacing tooth of the Marsupials was definitely regarded as representing/^. 4 of the Placental series, and was accordingly termed the fourth premolar ; the second tooth of that series being regarded as missing in the modern Marsupials. In this communication the author suggested the use of the term " milk-premolars,'" in lieu of milk-molars. Mr. Thomas's nomenclature of the Marsupial series was adopted in the ' Study of Mammals.' It was some years after the appearance of the paper last referred to that the researches of Messrs. Kiikenthal and Rose afforded grounds for regarding all the teeth in advance of the replacing premolar of modern Marsupials as milk-teeth, and the identification of tbe true molar series as corresponding serially with the milk set rather than with the premolars. To these discoveries I need not refer further than to say that a useful summary of them is given by Professor Osborn in the 'American Naturalist' for 18933. I accordingly pass on to two papers by Senor Florentino Ameghino, in the course of which the remains of certain Marsupiallike Mammals from the Tertiaries of Patagonia are described and figured under the group-name of " Sparassodonta." In the first of these communications 4 the animals in question are said to be referable neither to the Carnivora Vera, the Creodontia, or the 1 ' Le3 Enchancements, etc.-Mammiferes Tertiaires,' pp. 13-19. 2 Phil. Trans. 1887, p. 147. Many of the views propounded here were modified in a paper published in the Ann. Mag. Nat. Hist. ser. 6, vol. ix. p. 308 (1892). 3 Vol. xxvii. pp. 493-508. * Bol. Ac. Cordoba, vol. xiii. pp. 259-452 (1894). |
