x EVOLUTION OF THE HORSE 247 
the molar teeth is, too, a little different from that of Hqwus. Proto- 
hippus of the North American Pliocene is also three-toed, but the 
two additionally-developed toes are smaller than in Hipparion. 
Other forms are dealt with below in connexion with the ancestry 
of Perissodactyles. It is a curious fact about Hipparion, which 
is not now regarded as on the direct line of equine descent, that 
the edges of the enamel plates of the molars may show a com- 
plicated folding very like that presented by that clearly terminal 
form of Perissodactyle life, the gigantic Hlasmotherium. This is 
indicative of high specialisation, which ended in extinction. . 
Ancestry of the Horses.—The Lophiodontidae and the 
Palaeotheriidae are two of the most interesting extinct families 
of Perissodactyles; for among them we find what would appear 
to be the ancestral forms of both the existing Tapirs and Horses. 
The Rhinoceroses also would seem to be derivable from the 
Palaeotheriidae. The very vagueness of the characters of these 
creatures, considered from a classificatory point of view, has led to 
much diversity in their placing. This though gratifying to the 
evolutionist is tiresome to the writer who wishes to give a 
methodical account of their various characters. It will be best 
perhaps not to attempt an accurate placing or to reconcile con- 
flicting opinion, but to give some salient features of osteology 
which lead to the belief in their relationship to existing groups of 
Perissodactyles. A book upon the history of mammals would be 
incomplete without some account of that well-ascertained series of 
forms which seem to connect these primitive Perissodactyles with 
the modern Horse. Hguwus, in fact, is not only the “show horse ” 
of the doctrine of evolution, but also the “ stalking horse.” 
In the Eocene of both Europe and America are met with a 
number of forms from which we may start. Hyracotherium, 
regarded on the one hand as the type of a sub-family of the 
Equidae themselves, and on the other as a member of the family 
Lophiodontidae, was a small-sized animal, three feet or so in 
length ; it possesses the complete Eutherian dentition with a slight 
diastema. » The orbits are not separated from the temporal fossa ; 
the fore-limbs were four-toed, the hind three-toed, with moderately 
lone metapodia, especially on the hind-feet. The shoulder blade 
Soc. 1900, p. 379) in the cave which produced the remains of Glossotheriwm. A 
piece of skin covered with Fox-red hair, possibly spotted with paler areas, is believed 
to be a relic of Onohippidiwm. 
