Zoe ANCIENT RHINOCEROSES CHAP. 
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 ikeness 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 ' are the Hyracodonts and 
the Amynodonts. They both date from the Eocene, and became 
extinct in the succeeding Oligocene. Hyracodon” (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 Rhinoceroses. 
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 
13 Ci Pms M3. 
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 1s 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, 7riplopus, 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. ii. 1898. 
2 Scott, in Gegenbaur’s Festschrift, ii. 1896, p. 351. 
