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Show 1899.] THE MARSUPIAL AND PLACENTAL CARNIVORA. 925 Dasyuridce, although stated to present resemblances to each of these groups. Apparently in all cases the palate is devoid of the tin-ossified vacuities characteristic of existing Marsupials. In many instances the upper incisors exceed the number occurring in modern Placentals, one of the genera (Prothylacinus) having the same incisive formula as in Thylacinus, namely |. The cheek-teeth (as shown in figs. 3, 5, and 6 of Plate LXII.) are also of a Marsupial type, the total number being seven, of which the last four are molariform. And in his first communication Senor Ameghino divides them according to the formula usually accepted for the Marsupialia ; that is to say, into three premolars (p. 2, p. 3, and p. 4) and four molars. H e also goes on to observe that while the milk-dentition is more reduced than in the Carnivora, it is less so than in the Dasyuridce. The genus in which the reduction is carried to the greatest extent is the one named Borhyceua (Plate LXII. fig. 3), in which only the canine and the fourth cheektooth have vertical successors. On the other hand, in the other genera (e. g. Prothylacinus, fig. 5, and Amphiproviverra, fig. 6), both the second and third cheek-teeth, in addition to the canines, are thus replaced. In regard to the incisors there is no evidence. In the drawing (Plate LXII.) I have had the lower jaws of the three genera mentioned figured alongside of those of the Creodont genera Hycenodon (fig. 1) and Pterodon (fig. 2) above, and of the Marsupial Thylacinus (fig. 4) below. And an inspection of these will show that, whereas the jaws shown in figs. 1 and 2 have but three molariform teeth, all the others have four. The general resemblance is, however, so striking between the whole series, that it is almost impossible to conceive that the seven cheek-teeth are not serially homologous with one another in the six genera. And this idea has been developed in Seiior Ameghino's second paper, published in the Society's 'Proceedings' for the present year'. Thus on page 556 he writes that he assigns 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 in Marsupials) and to the second in others (e. g. the fourth in Placentals). This view is in fact the one advanced by Owen, when he said that the fourth cheek-tooth of the Thylacine was a milk-molar rendered permanent by the suppression of its vertical successor. And looking at the number of forms described by Senor Ameghino which serve in some degree to connect the Creodontia with the Dasyuridce, it appears to me, as already indicated, impossible to avoid accepting the above interpretation. The fourth cheek-tooth in the Prothylacinidos (Sparassodonta) indisputably belongs to the 1 Supra, pp. 555-571; I am not prepared to admit the Cretaceous age of some of the specimens described therein. 2 This nomenclature had been long since proposed by Dr. H. Winge, Vidensk. Med. Kjobenhavn, 1882, p. 65. PKOC ZOOL. Soc-1899, No. LX. 60 |