276 P. N. Bose— History of the Extinct Carnivora. 
either Mesonyx or Synoplotherium. The upper teeth of none of 
these genera are known, except the incisors, canines, and two pre- 
molars of Synoplotherium. They all agree, however, in not having 
any one tooth in the lower jaw differentiated out as the sectorial ; 
they all have a series of flesh-cutting teeth. But the exact form of 
these is not yet known. 
The'nearest approximation to the teeth of the Proviverroid and 
Hyenodont forms in the group of the Insectivora is presented by 
Centetes, the molars of which are similar in form, slightly increase 
in size from the first to the third, and are constructed on essentially 
the same type as the molars of Proviverra—there are three pointed 
cusps in front and a horizontal heel behind; and the step from 
Proviverra to Hyenodon and Pierodon is an easy one. There is no 
doubt that the teeth of these two last-mentioned genera have a 
striking resemblance to those of Thylacinus, though there are 
important points of difference with regard to the form of the 
teeth, etc. There is, however, one serious objection to the affiliation 
of the Hyenodonts with the Didelphide. M. H. Filhol’ has been 
fortunate in obtaining several beautifully preserved specimens 
showing the replacement of the deciduous by the permanent teeth. 
In the upper jaw four permanent molars (behind the Ist pm.) 
succeed as many milk ones; and in the lower jaw all the teeth of 
the adult dentition, except the hindermost carnassial, have prede- 
cessors; so that according to their development there are, strictly 
speaking, 5 premolars in the upper, and 6 in the lower jaw. 
Though such a large number of what must be regarded as talse 
molars is somewhat exceptional amongst the Placentals (certainly 
among the Carnivores), the deciduous dentition of Hyenodonts is 
quite unlike what obtains in the Marsupialia. 
In these, as has been shown by Prof. Flower, only one tooth of 
the permanent dentition (pm. 3) has a predecessor. 
(C.)—Summary.—The results of this section may be summarized 
as follows :— 
1. That starting from the Arctocyon, the most primitive Carnivore 
known to us, we have two divergent series, one comprising 
Paleonictis, Amphicyon and Cynodon; and the other, Proviverra, 
Hyenodon, Pierodon, Ambloctonus, Oxycena, and probably also Synoplo- 
therium, Mesonyx, Patriofelis, and Sinopa. 
2. That the first of these two series approach in the form of their 
teeth the typical Carnivora, of which, as we shall try to show here- 
after, they were the ancestors. 
3. That the second series formed an exceptional group of Carni- 
vores, of which Hyenodon was the last and most highly organized 
form. 
4, That the peculiarities in the dentition of the Eocene Carnivores 
can be most consistently explained by observing the modifications 
in the dentition of the Insectivora. 
§ 3.—Brain and Osteology. 
As a very general rule the brain cavity of the Eocene Carnivores 
1 op. cit. p. 169. 
