SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 23 



cubital arcade. The radius (PL IV, figs. 7, 7«) is relatively slender ; its 

 head, though occupying the whole width of the humeral trochlea, is not 

 broad, and has a transversely oval, antero-posteriorly compressed shape ; 

 on the inner side of the head is a small, oblique facet, which abuts against 

 the internal flange of the humeral trochlea ; this facet is continuous with 

 the proximal articular surface, which is composed of a low internal con- 

 vexity and a very shallow external concavity. The proximal ulnar facet 

 is very small and is confined to the postero-internal side of the head. On 

 the internal side of the shaft, just below the head, is the large and rugose, but 

 low, bicipital tubercle. The shaft is moderately arched forward and inward 

 and is of transversely oval cross-section, broadening slightly to the distal 

 end. The distal end is quite heavy, with a single broad and shallow tendinal 

 sulcus on the dorsal surface ; there is a small facet for the ulna on the 

 external side. The facets for the scaphoid and lunar are sharply demar- 

 cated by a ridge and are concave dorsally, becoming convex toward the 

 palmar side ; the lunar facet is reflected farther upward upon the palmar 

 aspect of the bone than is that for the scaphoid. 



In view of the degree of digital reduction in the manus, which is func- 

 tionally all but monodactyl, the ulna (PI. IV, fig. 8) is surprisingly large 

 and heavy, relatively quite as stout as in the tapir. The olecranon is well 

 developed and is much thickened and rugose at the free end ; it projects 

 upward and backward, extending well behind the plane of the shaft ; the 

 coronoid process is very prominent and the -sigmoid notch quite deep, 

 though short proximo-distally, its articular surface is continuous, but dis- 

 tally the external portion is produced considerably farther, extending 

 around the head of the radius, for which it has a small facet. The shaft 

 is strongly arched forward and very heavy, of subtrihedral cross-section and 

 contracting slightly to the distal end. The distal facet for the radius is a 

 small, but prominent oval convexity ; the pyramidal facet is relatively large 

 and simply convex, while that for the pisiform is exceedingly small. 



As Ameghino has shown (94<5, p. 266) the manus (PL V, figs, i, la, 1^,3,4) 

 has undergone a remarkable degree of reduction and bears a striking, 

 though entirely superficial, likeness to that of one of the three-toed horses, 

 such as ProtoJiippus, for example. The carpus is not at all equine, or even 

 perissodactyl, in character and may be defined as semi-taxeopod, the 

 scaphoid articulating with the magnum, but the lunar not coming into 

 contact with the unciform. In the carpo-metacarpal connections, this 



