IT DENTITION OF SPARASSODONTS 55 



Their correspondence with the milk series is shown in an 

 interesting way by the close resemblance which the last milk 

 premolar often bears to the first molar. These two extremes of 

 dentition, i.e. purely monophyodont and, excepting for the molars, 

 purely diphyodont, are however connected by an intermediate state 

 of affairs, which is represented by more than one stage. In 

 Borhyacna (probably a Sparassodont) the incisors and the canines 

 and two out of the four premolars belong to the permanent 

 dentition, while the two remaining premolars and of course the 

 three molars are of the milk series. Prothylacinus, a genus 

 belonging to the same group, has a dentition which is a step or 

 two further advanced in the direction of the recent Marsupials. 

 We lind, according to Ameghino, 1 whose conclusions are accepted 

 by Mr. Lydekker, that the incisors, canines, and two premolars 

 belong to the milk series, while the permanent series is repre- 

 sented only by the two remaining premolars. We can tabulate 

 this series as follows : 



(1) Purely monophyodont, with teeth only of the first set 

 Toothed Whales. 



(2) Incompletely monophyodont, as in the Marsupials, where 

 there is a milk dentition with only one tooth replaced. 2 



(3) Incompletely diphyodont, with the dentition made up 

 partly of milk, partly of permanent teeth, as in Borliyaena. 



(4) Diphyodont, where all the teeth except the molars are 

 of the second set ; this characterises nearly all the mammals. 



As we pass from older forms to their more recent representa- 

 tives there is as a rule a progressive development of the form of 

 the teeth. This is especially marked among the Ungulata. The 

 extremely complicated type of tooth found in such a form as the 

 existing Horse can be traced back through a series of stages to a 

 tootli in which the crown is marked by a few separated tubercles 

 or cusps. Arrived at this point, the differences between the teeth 

 of ancestral Horses and ancestral Ehinoceroses and Tapirs are hard 

 to distinguish with accuracy ; and the same difficulty is experi- 

 enced in attempting to give a definition of other large orders by 

 the characters of the teeth, such as will apply to the Eocene or 



1 Proc. Zool. Soc. 1899, p. 922. 



2 Mr. M. Woodward, however (P.Z.S. 1893, p. 467), is disposed to think that in 

 some Macropodidae at any rate the supposed tooth of the second set really belongs 

 to the milk dentition, arising late between Pra 3 and Pm 4 . 



