x DEFINITION OF GENUS EQUUS 239 



we are to continue the plan of defining the various families of 

 Mammalia. 



The genus Equus 1 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 Ehinoceros 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, i.e. forty -four. In 

 the skull the orbit is as it is not in Tapirs and Ehinoceroses 

 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, E. 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 



1 Sir W. H. Flower, The Horse, London, 1890. 



