292 UNGULATA 



EXTINCT TRANSITIONAL ARTIODACTYLES. 



In this place it will be convenient to notice briefly a few of 

 the extinct types of Tertiary Artiodactyles which connect the 

 existing bunodont Suina with the more specialised selenodont 

 groups mentioned below so closely as to show that in a strictly 

 palaeontological classification such groups cannot be maintained. 

 It should be mentioned that while some of these extinct forms 

 were in all probability actual ancestral links between the bun- 

 odonts and selenodonts, others, like the Anoplotheres, died out 

 entirely without giving rise to any more specialised descendants. 



Chceropotamidce. In this family the molars are intermediate in 

 structure between those of the Suidce and the next family. The 

 upper ones have very broad crowns, with the five columns arranged 

 as in Anthracotherium ; while the premolars are not secant, and may 

 be very large. The best known forms are the small Cebochozrus of 

 the Phosphorites of Central France ; Cfueropotamus of the Upper 

 Eocene, the type species of which was of the size of a large Pig, 

 with the dental formula if, c ^, p , m f , and no distinctly 

 selenodont structure in the molars the much larger Elotherium, 

 from the Upper Eocene and Lower Miocene of both the Old and New 

 Worlds, which presents the very rare feature of the absence of a third 

 lobe to the last lower molar ; and the equally large Tetraconodon of 

 the Pliocene of India, in which this third lobe was present and the 

 premolars were of enormous size. The remarkable North American 

 Eocene genus Achcenodon should perhaps also be placed here. 



Anthracotheriidce. The genera Anthracotherium and Hyopotamus, 

 of the upper Eocene and Miocene, 

 have the typical Eutherian dental for- 

 mula ; the upper molars (Fig. Ill) 

 carrying three columns on the anterior 

 and two on the posterior half of the 

 crown, all of which are of a more or 

 less decidedly selenodont structure. 

 The mandible has a descending flange 

 at the angle. The figured tooth (in 

 which the antero-internal and antero- 

 median columns are imperfect) may be 

 compared with the diagram given in 

 _ Fig. 5, p. 32, when the homology of 



Fio.in.-The imperfect third left tne columns or tubercles will be at 

 upper molar of Hyopotamus giganteus, once apparent, the broken antero- 

 JJJSiSJ* (Fr ' " median column representing the proto- 



conule. Some of the species are of 

 large size, while others are comparatively small. 



