ANCIENT RHINOCEROSES 



are reduced to five. The lower incisors are only two. The 

 sagittal crest is less marked ; the fifth digit is reduced to a tiny 

 nodule representing the metacarpus. It had a small nasal horn. 

 There are numerous other details of likeness to modern Rhinoceroses 

 in this creature, which has only community of descent, with them 

 from the older hornless forms, such as Aceratherium and Caenopus. 

 In the genus Peraceras the upper incisors are as completely gone 

 as in the living African Rhinoceroses. 



The most ancient rhinocerotine types 1 are the Hyracodonts and 

 the Amynodonts. They both date from the Eocene, and became 

 extinct in the succeeding Oligocene. Hyracodon 2 (Fig. 134) was 

 " an agile, light-chested, and rather long-necked " type, resembling a 

 Horse in build. There were no horns present, but the hoofs were 

 more like those of the Horses than of the existing Ehinoceroses. 

 These animals were apparently plain dwellers and defenceless, which 

 is held to account for their compact hoofs and outward similarity 

 to a Horse. The genus is Oligocene. The dental formula is 

 IfCi-Pmf Mf. 



It is surmised by Professor Scott that the number of dorso- 

 lumbar vertebrae was twenty-three or twenty-four. The radius and 

 ulna are complete and separate bones, but the latter is somewhat 

 reduced. There are four metacarpal bones, of which, however, the 

 fifth is much reduced. The animal is only three-fingered. The tibia 

 and the fibula are distinct, and show no tendencies towards fusion ; 

 but the fibula is much reduced. There are only three metatarsals 

 and three toes. Had this line, which is to be regarded as a side 

 branch of the Rhinoceros stem, not died out, it would probably 

 have resulted, thinks Professor Scott, in monodactyle very Horse- 

 like types. It is later than the next genus to be described, 

 Hyrachyus, of which it is possibly a descendant. An intermediate 

 type, Triplopus, appears to bind together Hyracodon and Hy- 

 rachyus. 



In Hyrachyus agrarius the skull is long and narrow, the 

 facial region being markedly longer than in existing Rhinoceroses. 

 The mastoid portion of the periotic bone is widely exposed upon 

 the outer face of the skull, which is, as has been said, not the 

 case with the existing genus Rhinoceros. The dentition is the 

 complete Eutherian dentition of forty -four teeth. The upper 



1 See Osborn, Mem. American Mus. Nat. Hist. vol. i. pt. iii. 1898. 

 2 Scott, in Gegenbaur's Festschrift, ii. 1896. p. 351. 



