1893. SUCCESSION OF TEETH IN MAMMALS. 363 
The only possible conclusion to be drawn is that in Didelphys 
(and probably in other Marsupials) the teeth of the permanent 
dentition, with the exception of the last premolar, represent the 
deciduous teeth of other forms, while rudiments of a second series 
are present in the embryo, but only cut the gum in the case of the third 
premolar ; these rudiments must obviously be the remnants of a more 
complete replacement. 
If the two series of teeth in the Mammalia are to be regarded as 
relics of the many series in the Reptilia, the Marsupials, inasmuch 
as they have almost lost one of these sets, have, in this respect at 
least, departed further from the common stock of Mammalia than 
have many of the true placental forms. There is, however, not 
unfrequently, considerable variation in the extent of replacement, 
even within the same groups of placental Mammals; for instance, 
among the Rodents, the Leporide have their incisors replaced, while 
the rat is entirely monophyodont. 
A further point of interest lies in the fact that, whereas in the 
Marsupials it is the second series of teeth, or that which corresponds 
with the permanent series of most higher forms, which has become 
degenerate, in other groups it is the precursory ones that are sup- 
pressed. We see this suppression of the milk teeth in some of the 
Carnivora: in the dog, for example, the deciduous teeth are well 
developed, the bear has much smaller ones, while in the seal they 
are still more rudimentary, and absorbed before birth. 
EWG: Pomrarp. 
