Mr. W.H. Flower on the Teeth in the Marsupialia. 183 
appears almost paradoxical to suggest that the milk- or deciduous 
teeth may rather be a set superadded to supply the temporary needs 
of mammals of more complex dental organization. But it should be 
remembered that, instead of there being any such relation between 
the permanent and the milk-teeth as that expressed by the terms 
*‘progeny”’ and “parent”’ (sometimes applied to them), they are 
both (if all recent researches into their earlier development can be 
trusted) formed side by side from independent portions of the pri- 
mitive dental groove, and may rather be compared to twin brothers, 
one of which, destined for early functional activity, proceeds rapidly 
in its development, while the other makes little progress until the 
time approaches when it is called upon to take the place of its more 
precocious locum tenens. 
Many facts appear to point to the milk-teeth as being the less 
constant and important of the two sets developed in diphyodont 
dentition. Among these the most striking is the frequent occurrence 
of this set in a rudimentary and functionless or, as it were, partially 
developed state. The milk-premolars of some Rodents (as the 
Guinea-pig), shed while the animal is in utero, the simple structure 
and evanescent nature of the milk-teeth of the Bats, Insectivores, and 
Seals, the diminutive first incisors of the Dugongs and Elephants, 
all appear to be cases in point. On the other hand, examples of 
the commencing or sketching out, as it were, of the successors to a 
well-formed, regular, and functional first set of teeth, are rarely, if 
ever, met with. Occasional instances of the habitual early deca- 
dence, or, perhaps, absence of some of the second or so-called per- 
manent teeth occur in certain animals ; but these are rather examples 
of the disappearance or suppression_of organs of which there is no 
need in the economy, and chiefly occur in isolated and highly medi- 
fied members of groups in the other members of which the same 
phenomenon does not occur, as in Cheiromys among the Lemurs, 
Trichechus among the Seals, and the recent Elephants (as regards 
the premolars) among the Proboscideans. They form no parallel 
to the cases mentioned above of the rudimentary formation of an 
entire series of teeth of the temporary or milk-set. 
To return to the Marsupials :—If this view be correct, I should be 
quite prepared to find, in phases of development earlier than those 
_ yet examined, some traces either of the papillary, follicular, or sac- 
cular stages of milk-predecessors to other of the teeth besides those 
determinate four in which, for some reason at present unexplained, 
they arrive at a more mature growth*. Such proof as this would 
alone decide the truth of these speculations ; and I have not at pre- 
sent either the requisite leisure or materials for following out so 
delicate an investigation. I trust that the facts already elicited are 
sufficiently novel and important to justify my bringing them, as they 
now stand, before the Society. 
* It may be remarked that the milk-tooth which alone is developed in the 
Marsupials corresponds homologically with that which, as a general rule, is most 
persistent in the typical diphyodonts, including Man, viz. the posterior milk- 
molar, replaced by the posterior permanent premolar. 
