158 New Equine MamiivtU from the Tertiary Formation. 



with considerable cei'tainty, from the thirty or more well marked 

 species that lived in the intervening periods. The natural line of de- 

 scent would seem to be through the following genera : Orohippus, of 

 the Eocene ; Miohippm and Anchitherium, of the Miocene ; Anchippm 

 Hipparion Protohippm and Pliohippusy of the Pliocene; and Equu-f, 

 Quaternary and recent. 



The most marked changes undergone by the successive equine genera 

 are as follows : 1st, increase in size ; 2d, increase- in speed, through 

 concentration of limb bones ; 3d, elongation of head and neck, and 

 modifications of skull. The increase in size is remarkable. The 

 Elocene Orohippus was about the size of a fox. Miohippus find Anchither- 

 ium, from the Miocene, were about as large as a sheep. Hipparion 

 and Pliohippus, of the Pliocene, equaled the ass in height ; while the 

 size of the Quaternary Eqnus was fully up to that of the modern horse. 

 The increase of speed was equally marked, and was a direct result 

 of the gradual modification of the limbs. The latter were slowly con- 

 centrated by the reduction of their lateral elements and enlargement 

 of the axial one, until the force exerted by each limb came to act 

 directly through its axis, in the line of motion. This concentration is 

 well seen, e. g., in the fore limb. There was, 1st, a change in the 

 scapula and humerus, especially in the latter, which facilitated motion 

 in one line only ; 2d, an expansion of the radius and reduction of the 

 ulna, until the former alone remained entire and effective; 3d, a short- 

 ening of all the carpal bones, and enlargement of the median ones, 

 insuring a firmer wrist ; 4th, an increase hi size of the third digit, at 

 the expense of those on each side, until the former alone supported 

 the limb. The latter change is clearly shown in the following dia- 

 gram (here omitted), which represents the fore feet of four typical 

 genera in the equine series, taken in succession from each of the geo- 

 logical periods in which this groupof mammals is known to have lived. 

 The ancient Orohippus had all four digits of the manus well developed. 

 In Miohippus, of the next period, the fifth toe has disappeared, 

 or is only represented by a rudiment, and the limb is supported 

 by the second, third, and fourth, the middle one being the largest. 

 Hipparion, of the later Tertiary, still has three digits, but the third 

 is much stouter, and the outer ones have ceased to be of use, as they 

 do not touch the ground. In Equus, the last of the series, the 

 lateral hoofs are gone, and the digits themselves are represented only 

 by the rudimentary splint-bones (the modern horse occasionally has 

 one of the ancestral hooflets developed usually on the forefoot). The 

 middle or third digit supports the limb, and its size has increased ac- 

 cordingly. The corresponding changes in the posterior limb of these 



