HIPPOPOTAMUS. 33 



that Acotheruhim, one of the Chceromoridas, a group of primitive pigs, may indi- 

 cate the source from which the hippopotami were derived. This form occurs in 

 the Upper Eocene phosphorites of France. Osborn 1 has remarked on the affinity 

 to the hippopotami of Merycopotamus, an animal originally described as a hippo- 

 potamus by Falconer and Cautley, but now placed near the Anthracotheres. 



The situation of the region where the evolution of the hippopotami took place 

 is unknown, but in view of the complete absence from the European Oligocene and. 

 Miocene deposits of ancestral forms, it must have occurred outside Europe. 

 Stehlin and Forsyth Major' 2 have suggested that the hippopotami, like the elephants, 

 originated in Africa. 



Fig. 17.— A left astragalus seen from behind. 3 natural size, a a, surfaces for articulation with 

 caleant'um ; 6, with cuboid ; c, with navicular, g right calcaneum seen from behind, d, surface for 

 articulation with cuboid. 



The hippopotami suddenly appear in great variety in the Lower Pliocene beds 

 of India, and by late Pliocene times had reached northern Africa and western 

 Europe. A small hexaprotodont hippopotamus was described by Pan tanelli 3 from 

 strata probably of Lower Pliocene age from Casino near Siena. 



The Indian species form a remarkable group and their affinities have been 

 fully discussed by Lydekker. 4 He points out that specialisation has taken place 

 along two main lines : (1) the shortening and widening of the mandibular symphysis 

 frequently accompanied by a general shortening of the cranium and mandible; 

 (2) the reduction in number of the incisors, this reduction probably occurring first 



1 ' Age of Mammals,' p. 313. 2 ' Geol. Mag.' [4], ix (1902), p. 197. 



3 ' Atti R. Accad. Liucei ' [3], iii (1879), p. 318. 

 * ' Palseont. Ind.,' ser. 10, iii, p. 47. 



5 



