132 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



ing well down upon the dorsal face, and becoming slightly concave near 

 the palmar side. The facet for the pyramidal is very large and somewhat 

 concave, but is confined to the distal moiety of the ulnar face ; proximally, 

 the two bones are quite separate and not in contact (see PI. XIX, fig. 6a). 

 Distally, the lunar rests only on the magnum and is quite widely separated 

 from the unciform by the interposition of the pyramidal. On this side, the 

 carpus is strictly taxeopod in the arrangement of its elements. 



The pyramidal is relatively large and has a greater proximo-distal height 

 than any of the other carpals. Proximally, this bone is rather shallow 

 palmo-dorsally, but becomes very thick in this dimension toward the distal 

 end, and a blunt, rugose process projects from the palmar side. The ulnar 

 facet is very large and saddle-shaped and that for the pisiform, which is 

 also large, presents internally as much as toward the palmar side. On 

 the radial side, near the distal end, is a large surface for the lunar and, 

 distal to this, a narrow facet for articulation with the magnum. This con- 

 tact is, however, entirely lateral, the pyramidal not overlapping the magnum 

 at all. Here we find a difference from the carpus of Macrmtchenia, in 

 which the pyramidal extends over upon the magnum and thus forms 

 another departure from the serial arrangement. (See Gervais, '55, PI. 8, 

 fig. 2.) 



The pisiform is short, heavy, of irregular shape and much thickened at 

 the free end, but nearly uniform in the other diameter. The proximal end 

 is also thickened and bears a high, narrow and somewhat saddle-shaped 

 surface for the pyramidal, and on the external side a very small plane 

 facet for the ulna. In Macrauchenia, the pisiform is of similar general 

 shape to that of Theosodon, but is more compressed laterally, and con- 

 tracts to form a very much more slender proximal end. 



The trapezium is much reduced in size and was of no great functional 

 importance. Proximo-distally, it is elongate, tapering to a bluntly rounded 

 point at the distal end, but the transverse and dorso-palmar diameters are 

 small. The proximal end is a small, simply convex surface for articula- 

 tion with the scaphoid, while on the ulnar side are two facets, one for the 

 trapezoid and, below this, a much larger concavity for the second meta- 

 carpal. The trapezoid I have not seen, though it may be readily recon- 

 structed from the space left vacant between the surrounding bones, which 

 shows it to have been nearly as large as the magnum. This element has 

 been figured by Ameghino ('94(5, 282, fig. 18). 



