466 



Mr. W. H. Flower on the Development [^^^y 9, 



to the incisor teeth. It is questionable whether the first premolar of those 

 animals of this group which have four premolar teeth, as the Hog, Dog (man- 

 dible), &c., ever has a deciduous predecessor, at all events so far advanced 

 as to have reached the calcified stage. But the closest analogy with the 

 marsupial mode of succession is found among the Rodents. Here the incisors 

 appear to have no deciduous predecessors ; and in the Beaver, Porcupine, 

 and others, which have but four teeth of the molar series, i. e. three true 

 molars and one premolar, the latter is, exactly as in the Marsupials, the 

 only tooth which succeeds a deciduous tooth. The analogy, however, does 

 not hold in those Rodents which have more than one premolar, as the Hare ; 

 for in this case each of these teeth has its deciduous predecessor. 



In the preceding account I have used the term "permanent " for those 

 teeth which remain in use throughout the animal's life, or, if they fall out 

 (as do the rudimentary canines and the premolars of the Macropodidce), 

 do not give place to successional teeth ; and I have therefore assumed that 

 the milk or temporary dentition of the typical diphyodont mammals is re- 

 presented in the Marsupials only by the deciduous molars. It may be held, 

 on the other hand, that the large majority of the teeth of the Marsupials are 

 the homologues of the milk or first teeth of the diphyodonts, and that it is 

 the permanent or second dentition which is so feebly represented by the 

 four successional premolars. This view is supported by many general 

 analogies in animal organization and development, such as the fact that the 

 permanent state of organs of lower animals often represents the immature 

 or transitional condition of the same parts in beings of higher organization. 



Looking only to the period of development of the different teeth in some 

 of the marsupial genera, we might certainly be disposed to place the suc- 

 cessional premolar in a series by itself, although, indeed, all its morphological 

 characters point out its congruity with the row of teeth among which it 

 ultimately takes its place, the reverse being the case with its predecessor. 

 It is, however, almost impossible, after examining the teeth of the young 

 Thylacine described and figured in the paper, to resist the conclusion 

 originally suggested. The unbroken series of incisors, canines, premolars, 

 and anterior true molars of nearly the same phase of development, with 

 posterior molars gradually added as age advances, form a striking contrast 

 to the temporary molar, so rudimental in size, and transient in duration. 

 I can scarcely doubt that the true molars of this animal would be iden- 

 tified by every one as homologous with the true molars of the diphyodonts, 

 which are generally regarded as belonging to the permanent series, although 

 they never have deciduous predecessors. Now, if the homology between 

 the true molars of the Thylacine and those of a Dog, for instance, be 

 granted, and if the anterior teeth (incisors, canines, and premolars) of the 

 Thylacine be of the same series as its own true molars, they must also be 

 homologous with the corresponding permanent teeth of the Dog. 



It may be objected to this argument, that the true molars of the diphyo- 

 donts, not being successional teeth, ought to be regarded as members of the 



