In the typical diphyodont dentition, such as is exhibited for example in Man and the vast majority of mammals, the milk teeth eventually completely disappear and are entirely replaced by the permanent set of teeth, with the exception, of course, of the molars, which though they are developed late belong to the milk series. {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 _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,[28] whose conclusions are accepted by Mr. Lydekker, that the incisors, canines, and two premolars belong to the milk series, while the permanent series is represented 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.[29] (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 mammals.