x DEFINITION OF GENUS ZQUUS 239 
we are to continue the plan of defining the various families of 
Mamunalia. 
The genus Aywus' contains not only the Horse, but the Asses 
and Zebras. The genus is to be distinguished as regards external 
characters by the following features: — The body is thickly 
clothed with hair; there is a more or less bushy tail and mane; 
the colours are apt to be disposed in stripes of black or blackish 
upon a yellowish brown ground; this is of course best seen in 
the Zebras, but the wild Asses also have some traces of it, if only 
in the single cross-bar of the African Wild Ass, and it is even 
“reversionary ” in the domestic Horse at times. There are no 
horns upon the forehead or elsewhere; the fore-limbs or both 
pairs have a callous pad upon the inside, which is possibly to be 
looked upon as an aborted gland, possibly originally of use as 
secreting some odorous substance calculated to enable strayed 
members of the herd to regain their companions. The terminal 
phalanx of each of the (functionally) single digits is enclosed in 
a large horny hoof. 
The main internal features of structure which divide this 
genus of Perissodactyles from the Rhinoceros or the Tapir, or 
from both, are: the existence of strong incisors, three on each 
side of each jaw; there are canines, but these are small and do 
not always persist in the full-grown mare. They are popularly 
known as “tusks” or “tushes.” The first of the four premolars 
(the “wolf tooth”) is small and quite rudimentary; it is often 
absent. As there are three molars, the present genus has the 
“typical” number of the Eutherian dentition, 7.c. forty-four. In 
the skull the orbit is—as it is not in Tapirs and Rhinoceroses— 
completely encircled by bone. There is but one functional finger 
and toe on each hand (Fig. 121 C) and foot ; the second and third 
digits are represented by mere splints, one of which may as an 
abnormality be enlarged, and reach nearly as far as the well- 
developed digit. There are even occasionally traces of digit 
number two. 
The Horse, #. caballus, is to be distinguished from its con- 
geners by the small callosities on the hind-limbs which it pos- 
sesses in addition to the larger ones on the fore-limbs. The 
hairy covering of the tail is more abundant, as is also the mane. 
The head too is proportionately smaller, and the general contour 
2) 
1 Sir W. H. Flower, The Horse, London, 1890. 
