134 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



The metacarpus consists of three functional members, mc. II, III and 

 IV, and a minute remnant of mc. V. In symmetry, the manus is mesax- 

 onic, or perissodactyl, the median plane bisecting the third digit, which is 

 symmetrical, while the lateral digits, II and IV, each asymmetrical in 

 itself, form a symmetrical pair. Functionally, the manus is almost iso- 

 dactyl, with three subequal digits. 



Metacarpal II is elongate and relatively heavy. The head is large and 

 carries four distinct articular facets ; on the radial side is quite a large, 

 convex surface for the trapezium and on the proximal end is the large, 

 irregular facet for the trapezoid. On the ulnar side, a prominent, heavy 

 projection abuts against the magnum and overlaps the head of mc. III. 

 This magnum facet is slightly oblique and that for mc. Ill is in two parts, 

 one presenting distally and the other toward the ulnar side ; the latter is 

 a small, oval concavity, placed near the dorsal border. The shaft is stout 

 and heavy, of trihedral section and curved so as to make the radial border 

 concave. The distal trochlea is low and the carina is continued around 

 upon the dorsal face, though it is much more prominent on the palmar 

 side ; where the dorsal and palmar portions of the keel meet, there is a 

 distinct notch. On the palmar face, a non-articular ridge extends upward 

 for some distance in continuation of this carina, of which it evidently acted 

 as a support, and gradually dies away upon the shaft. 



Metacarpal III is longer, but decidedly more slender than either mc. II 

 or IV. The head is quite heavy and is drawn out into a massive triangular 

 process on the palmar side ; the facet for mc. II extends across the whole 

 dorso-palmar diameter of the head and, for most of its length, is obliquely 

 proximal in position, but near the dorsal border it curves downward and 

 presents toward the radial side. The proximal surface for the magnum is 

 large and irregularly saddle-shaped. From the ulnar side is given off a 

 very short projection, which articulates with the unciform, but occupies 

 only the dorsal moiety of the head. On the ulnar side, below the unci- 

 form projection, are two facets for mc. IV, which are elongated proximo- 

 distally, but very narrow ; the one near the dorsal border is quite deeply 

 concave, the other near the palmar border is nearly plane. The shaft is 

 long, narrow, straight and symmetrical, and subquadrate in section, 

 broadening slightly near the distal end. Distally, the semicylindrical 

 trochlea is higher than that of mc. II or IV, but the carina is less promi- 

 nent on the dorsal face, indeed, is but faintly indicated, while on the pal- 



