IT DENTITION OF SPARASSODONTS 5 
UL 
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, z.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 
Borhyaena (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 find, according to Ameghino,’ 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” 
(3) Incompletely diphyodont, with the dentition made up 
partly of milk, partly of permanent teeth, as in Borhyaena. 
(4) Diphyodont, where all the teeth except the molars are 
of the second set ; this characterises nearly all the mamuuals. 
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 
tooth 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 Rhinoceroses 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 
x 
the characters of the teeth, such as will apply to the Eocene or 
l 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 Pm, and Pm,. 
