IV. 
The Succession of Teeth in Mammals. 
HE question as to the mode of origin of the successional teeth of 
Mammalia is one which has at various times received a con- 
siderable amount of attention from anatomists, and one which it may 
be of some interest to review in the light of recent researches by Dr. 
Kikenthal.? 
In many of the Mammalia, as is well known, there are two sets 
of teeth, the first, or milk teeth, which are usually deciduous, and 
followed by a second more permanent set. In numerous reptiles and 
fish, on the other hand, we find a perpetual succession of teeth. 
Instead of the sets being limited to two, there is a constant replace- 
ment; as fast as one series of teeth wears away a new series appears 
to take its place. Moreover, in these reptiles and fish the replace-,. 
ment is also more complete than in Mammalia, for whereas in the 
latter we never find the whole series of teeth replaced, the true molars 
being never preceded or followed by others, in the fish and reptiles, 
when there is replacement of one there is replacement of all the 
teeth. 
When the contrast between this diphyodont condition of 
Mammalia and the polyphyodont one of Reptilia is realised, the 
question most naturally arises, is the diphyodontism of the one a 
relic of the polyphyodontism of the other, or is it to be regarded as an 
advance from a monophyodont state, one in which a single set of 
teeth alone was possessed ? 
Two years ago much might have been said in favour of a 
primitive monophyodont condition by an appeal to the simplest 
Mammalia. In the first place, in the toothed whales, the Mammalia 
possessed of the most reptilian type of teeth and jaw, only one set of 
teeth was known. Again, in the Monotremes there is no replace- 
ment, and, indeed, the teeth which do exist are rarely used, being 
generally concealed beneath horny plates. Lastly, in the Marsupials 
only one tooth in each jaw, the posterior premolar, is ever replaced, 
and this seemed to give us the starting point for the more complete 
change occurring in higher forms. 
Dr. Kikenthal’s recent work has, however, invalidated two of 
1 Ann. Mag. Nat. Hist., 1892. 
