1382 : 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 Macropodide), 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 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 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 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 
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, ought 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 permanent series. 
We haye been so long accustomed to look upon the second set of 
teeth as an after-development or derivative from the first, that it 
