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Show 1899.] THE MARSUPIAL AND PLACENTAL CARNIVORA. 923 Placental Carnivora, as i. 3, c. 1, p. 4, m. 3 ; while he gives that of the Marsupial Thylacinus1 as i. 3, c. 1, p. 3, m. 4. Nothing is said as to any replacement in the dental series of the latter genus, or in Marsupials generally; the division of the cheek-series into premolars and molars having been apparently made solely from the form and characters of the teeth themselves. But it is important to recognize that the premolars and molars were regarded as being numerically just the reverse of one another in the D og and the Thylacine ; and that this view has been accepted by almost all subsequent writers till quite recently. In 1867 Sir William Flower2 carried matters one stage further by proving that, when any replacement at all occurred, only one pair of teeth in each jaw was changed in the modern Marsupials ; this pair being the third of the cheek-series of seven. It was further argued that this replacing pair of teeth corresponded to the fourth cheek-tooth of the Dog, thus indicating that one premolar tooth (the first) was wanting in the Marsupial cheek-series, and hence suggesting that the full series in that group was originally i. 3, c. 1, p. 4, m. 4. It is, however, noteworthy that the three premolars of the Thylacine were still called p. 1, p. 2, and p. 3; and that the same notation was retained in the article " Mammalia" by the same writer in the 9th edition of the ' Encyclopaedia Britannica.' By the date of the issue of the third volume of his ' Anatomy of Vertebrates' (1868), O w e n 3 had likewise recognized the fact that only a single pair of teeth were replaced in each jaw of the Marsupials ; this, lie said, " giving the extent of the theoretical deciduous series." Prom this it may be inferred that he did not accept the homology of the replacing tooth of the Marsupials with p. 4 of the Placental series. But in a later part of the second volume (pp. 378 & 379) occurs the following very remarkable statement, which, although not altogether an exact solution of the problem, makes a very near approach to it:-" The observed phenomena of the development and change of the teeth led to the generalisation that the Marsupial differed from the Placental Diphyodout Mammals in having four true molars, i. e. m. 4 instead of m. 3; aud also that they differed in having only three premolars, i. e. p. 3 instead of p. 4 ; the typical number of the grinding series, 7, being the same ; and it was convenient for comparison to symbolise them accordingly. Since, however, there is reason to conclude that m. 1 in the Placental Diphyodonts is a continuation of the deciduous series of molars, which might be symbolised as dm. 5, and only becomes a permanent molar because there is no premolar developed above it so we may regard the tooth marked m. 1 [that is to say, the fourth of the cheek-series] in Thylacinus as being an antecedent tooth of the deciduous series, rendered permanent by a like reason, the suppression of p. 4. In other words, that m. 1 in Thylacinus is the homologue of dm. 4 [the last milk-molar] of Sus [or Cams'], 1 Ibid. p. 377. 2 Phil. Trans. 1867, p. 631. 3 Page 285. |