FOSSIL MAMMALIA. 47 



zontal, broad and convex before, narrow and concave behind ; the outermost 

 facet is a small triangular surface inclined downwards to the level of the articu- 

 lating surface of the outermost metacarpal. It also presents a posterior vertical 

 articular surface for a sesamoid bone. The proximal extremity of the outer 

 metacarpal bone is joined to the middle metacarpal, not by one semilunar surface, 

 but by two separate articulations of small size (fig. 4 and 5, PL XV.) ; it presents 

 a single large slightly convex articular surface for the os magnum, of an irregular 

 semicircular form, with the convexity of the curve turned outwards. 



The metacarpus increases in breadth as it approaches the phalanges ; the 

 two lateral metacarpals bending slightly away from the middle one, and expand- 

 ing towards their distal extremities : the middle bone presents a symmetrical 

 figure except at its proximal extremity (fig. 2, PI. XI.) The distal articulating 

 facet of each of the metacarpal bones extends so far upon both the anterior and 

 posterior surfaces as to describe more than a semi-circle (fig. 3, PI. XT.) ; in the 

 two lateral metacarpals it is traversed throughout by a longitudinal convex ridge 

 dividing it into two equal lateral parts ; the ridge is most produced on the 

 posterior half of the joint (fig. 4, PI. XI.) : in the middle metacarpal this ridge 

 subsides before it reaches the anterior part of the articular surface. 



The proximal extremity of the middle proximal phalanx presents a posterior 

 notch corresponding to the above partially developed ridge : the proximal extre- 

 mities of the lateral phalanges are traversed by a middle longitudinal depression, 

 and two lateral shallow concavities (fig. 6, PI. XI. ;) but these are of such an 

 extent as to be in contact with only a part of the convexity above, which therefore 

 was doubtless adapted to a sesamoid bone on each side of the longitudinal ridge. 

 The structure of the above described joints proves that the motion of the toe 

 upon the metacarpus was much freer and more extensive than in the Rhino- 

 ceros, which is the only existing Ungulate mammal which presents the tridactyle 

 structure in the fore-foot. In this species the metacarpo-phalangeal articulations 

 exhibit only a slight trace of the longitudinal ridges and grooves which are con- 

 fined to the posterior part of the joint; these are more developed in the Camelidce ; 

 but the Hog and Horse in this respect approach nearer to the Macrauchene, though 

 the structure of the metacarpo-phalangeal joints in the Hog falls far short of the 

 compactness and strength combined with freedom of play in flexion and extension 

 which distinguish those of the Macrauchene. The Palceotherium medium most 

 resembles the Macrauchene in the structure of the trochlear metacarpo-phalan- 

 geal joints; but both in this species,* and the Pal. crassum-\ the articular surface 

 at the distal end of the metacarpal is relatively narrower than in the Macrau- 

 chenia; moreover all the species of the extinct Palseothere differ from the Macrau- 



* See Ossem. Fossiles, PI. XX. fig. 3. t Loc. cit. PI. XXII. fi°\ 6. 



