IV. 



The Succession of Teeth in Mammals. 



THE question as to the mode of origin of the successional teeth of 

 MammaUa 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 Hght of recent researches by Dr. 

 Kukenthal.i 



In many of the Mammaha, 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. Kiikenthal's recent work has, however, invalidated two of 



' Ann. Mag. Nat. Hist., 1892. 



