THE ANCIENT MAMMALS OF BRITAIN. 323 



don) from the Hordwell beds, by a second (Pterodon) from the 

 Bembridge limestone, and a third (Argillotherium) from the 

 London cla} r . The last is, however, only known by an imper- 

 fect skull without the teeth, and may prove to be identical with 

 one of the foreign genera. 



Passing on to the Hoofed Order, one of the most interesting 

 of the Even-toed section is the Dichodon of the Hordwell and 

 Headon beds, as being the only early British Ungulate having 

 the upper molar teeth with the four crescentic columns char- 

 acterising the true Ruminants, to the ancestors of which group 

 it was probably more or less closely allied. Connecting these 

 early Ruminant-like forms with the ancestors of the Pigs is the 

 Hyopotamus of the Hempsted beds, in which the upper molars 

 were of the same general type as those of the Anoplothere 

 (represented in Fig. A of our illustration, p. 324), although the 

 whole form of the creature was more Pig-like, the skull being 

 long and narrow, with large tusks, separated by an interval both 

 from the incisor teeth in front and the pre-molars behind. The 

 Hyopotamus was doubtless a four-toed animal like the Hippo- 

 potamus, but an apparently allied form from the Headon beds 

 takes its name of Diplopus from the reduction of, the toes to 

 two in each limb. In the allied Anthracotherium, of both the 

 Hempsted and Headon beds, the molars lose to a great extent 

 the crescentic structure of those of the Hyopotamus ; and in the 

 gigantic Elotherium from Hempsted, and the smaller Charo- 

 potamus of the Bembridge series, we come to Ungulates, having 

 tubercular molars of the same general type as those of the 

 Pigs, although in the upper jaw they retain the five-columned 

 arrangement characterising the Hyopotamus^ and have much 

 squarer crowns than those of the Pigs. There can, however, 

 be little doubt that in this group we are very close to the an- 

 cestral stock from which the modern Pigs and Ruminants have 

 alike originated. 



On the other hand, the Anoplotheres (Anoplotherium\ which 



y 2 



