440 SCIENCE PROGRESS. 



and especially from the tooth change in Tkylacinus, Flower 

 put forward the theory that the persistent teeth in the mar- 

 supials corresponded with the second dentition of the pla- 

 centalia, and that the milk teeth of the latter were a secon- 

 dary set superadded to the permanent ones and represented 

 in marsupials by a single tooth only, the "third" (fourth) 

 deciduous premolar. He further sought to demonstrate 

 (6) that the homodont dentition of the cetacea was homolo- 

 gous with the second or replacing dentition of the more 

 typical mammals. This last conclusion, which has been so 

 long accepted, was deduced from a comparison between the 

 dentitions of the Odontoceti and the Pinnipedia, the latter 

 possessing a very much reduced milk dentition and func- 

 tional teeth which have a tendency to become homodont. 



Those views were accepted in their entirety by Huxley 

 (8), and it was largely due to him that there was first formulated 

 the conception of affinity between the cetacea and the seals (9). 

 The comparison between the dentitions of these forms has 

 been now shown to be misleading (26), as the cetacea pos- 

 sess persistent milk teeth, with which, in some cases, there are 

 said to be fused vestigial replacing ones, while the pinnipedia 

 on the other hand possess a well-developed replacing set, their 

 milk set, even including the molars, being in some forms very 

 much reduced. 



Although the belief in the primary nature of the re- 

 placing teeth did not attain much support on the continent 

 it met with more success in England; and in 1887 Thomas 

 (10) contributed a very important paper in which he sup- 

 ported this view. In this paper he identified the solitary 

 changing premolar of the marsupials with the fourth pre- 

 molar of the placentalia and not with the third as had 

 hitherto been generally done, and he showed that it was 

 pm2 which was suppressed in all living marsupials. 

 Four premolars were thus supposed to have been charac- 

 teristic of the earliest mammals as well as of the later ones. 

 This view was supported by the presence of a replacing 

 tooth under the fourth cheek tooth of Triacanthodon, which 

 Thomas demonstrated for the first time. This interpretation, 

 although generally adopted, has been criticised by Cope 



