1899.] THE MAESUTIAL AND PLACEXTAL CAENITOEA. 923 



Placental Carnivora, as i. 3, c. 1, p. 4, ?)i. 3 ; while he gives that of 

 the Marsupial Tliylacinus^ 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 pi'emolars 

 and molars having been apparently made solely fe'om the foi'ni 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 Dog and the Thylacine ; and 

 that this view has been accepted by almost all subsequent writers 

 till quite recently. 



In 1867 Sir William Flower- 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 tliat this replacing pair of teeth corresponded to 

 the fourth cheek-tooth of the Dog, thus indicating that one pre- 

 molar 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 pre- 

 molars of the Thylacine were still called jj. 1,^?. 2, and p. 3; and 

 that the same notation was retained in the ai'ticle " Mammalia " 

 by the same writer in the 9th edition of the ' Encyclopaedia 

 Britannica.' 



By the date of the issue of the third volume oi his ' Anatomy 

 of Vertebrates ' (1868), Owen ^ had likewise recognized the fact 

 that only a single pair of teeth were replaced in each jaw of the 

 Marsupials ; this, he said, " giving the extent of the theoretical 

 deciduous series." From 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 Mar- 

 supial differed from the Placental Diphyodont Mammals in having 

 four true molars, i.e. in. 4 instead of m. 3; and also that they 

 differed in having only three premolars, i. e. p. 3 instead oip. 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 Pla- 

 cental 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 Canis], 

 1 Md. p. 377. ^ Phil. Trans. 1867, p. 631. 3 Page 285. 



