132 Royal Society : — 



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 MacropodidcB), do not give place to successional teeth ; and I 

 have therefore assumed that the milk or temporary dentition of the 

 typical diphyodont mammals is represented 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 INIarsupials 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 successional 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 Thylaciue 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 develoi)ment, 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 

 identified by every one as homologous with the true molars of the 

 diphyodonts, which are generally regarded as belonging to the per- 

 manent 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 

 diphyodonts, not being successional teeth, oiight to be regarded as 

 members of the first or milk-series ; but, in truth, the fact that they 

 have themselves no predecessors does not make them serially homo- 

 logous with the predecessors of the other teeth, while their morpho- 

 logical characters, as well as their habitual persistence throughout 

 life, range them with the second or })ermanent series. 



We have been so long accustomed to look upon the second set of 

 teeth as an after-development or derivative from the first, that it 



