SUCCESSION OF MAMMALIAN TEETH. 449 



lowly form of theromorphous reptile and not from the more 

 specialised Theriodontia. It is quite possible that the latter 

 and the Mammalia had a common ancestor ; and I think it is 

 not improbable that the teeth of that form instead of being 

 simple cones might already show traces of heterodontism. 



The process of evolution of the specialised heterodont 

 dentition of the Mammalia, or of the Theriodontia, from the 

 simple homodont and polyphyodont dentition of the lower 

 Reptilia would, I think, necessarily cause a reduction in 

 number of the successional sets of teeth, due to an enlarge- 

 ment of one set and a consequent abstraction of growth, 

 energy and material from the underlying sets. This 

 specialisation would not appear in the first generation of 

 teeth, which must necessarily be of small size from its 

 early development and consequent adaptation to the small 

 jaw of the young animal, and which would moreover be 

 required for temporary use, while the larger and more com- 

 plicated dentition was developing. The increased size and 

 specialisation of the second set of teeth might well ab- 

 stract the growth energy from the succeeding third and 

 fourth sets which will consequently become much re- 

 tarded and eventually cease calcifying. We find such stages 

 illustrated by Parieasaurus, where the first dentition has 

 probably been shed, the second is functional and the third 

 is becoming reduced, and in the theriodonts, where the 

 second dentition is still more specialised, no traces of the 

 development of a third set is known (50, 51). We may 

 thus provisionally conclude that the earliest mammalia may 

 have possessed only two calcified sets of teeth, one of these 

 being very slightly developed and quite temporary ; and 

 this tending to be still more-so as the descendants of these 

 animals developed their larger and more important hetero- 

 dont dentition earlier and earlier ; the process of reduction 

 would go on until this first (premilk) dentition became 

 either quite suppressed, as it is in the majority of mammals, 

 or so much reduced as to be only present in the foetus and 

 never cutting the gum as is the case in Myrmecobius and 

 possibly Phascologale. One very important feature con- 

 cerned in the suppression of this set of teeth in the early 



