26S THE MANUS. [cHAr. 



two distinct types, each characteristic of, and giving name 

 to, one of the sub-orders. 



1. The Perissodactyla^ or " odd-toed " Ungulates, have the 

 middle or third digit the longest, and symmetrical in itself, 

 the free border of the ungual phalanx being evenly rounded. 

 The second and fourth toes may be subequally developed, as 

 in the Rhinoceros (Fig. 94), or they may be represented 

 only by mere splint-like rudiments of their metacarpals, as in 

 the Horse (Fig. 95). All intermediate conditions are met 

 with in various extinct forms, as Pakcothei'iuvi^ Anchitheriiim . 

 and Hipparion. In the Tapir (Fig. 93) there are four 

 complete toes, in consequence of the fifth being developed, 

 though it scarcely reaches the ground in walking. In other 

 respects the foot resembles that of the Rhinoceros, the third 

 toe being longest, and symmetrical in itself, and having on 

 each side of it the nearly equal second and fourth. In the 

 Rhinoceros there is a rudiment only of the fifdi metacarpal. 



In the Horse (Fig. 95), the three bones of the first row of 

 the carpus are subequal. The second row consists of a 

 very broad and flat magnum (;;/), supporting the great third 

 metacarpal, having to its radial side the trapezoid {id), and 

 to its ulnar side the unciform (//) which are both small, and 

 articulate distally with the rudimentary second and fourth 

 metacarpals. The pisiform is large and prominent, flattened 

 and curved; it articulates partly to the cuneiform, and 

 partly to the lower end of the radius. The single digit 

 consists of a moderate-sized proximal, a very short middle, 

 and a wide, semilunar, ungual phalanx. There is a pair of 

 large nodular sesamoids behind the metacarpo-phalangeal 

 articulation, and a single, transversely extended, " navicular" 

 sesamoid behind the joint between the second and third 

 phalanx. 



2. The Artiodactyla have the third and fourth digits almost 



