130 PATAGONIAN EXPEDITIONS: PAL^eONTOLOGY. 



the radius and reduction of the ulna are remarkably small, when compared 

 with Artiodactyla or Perissodactyla of a similar degree of specialization. 

 The head is very small (see fig. 3a), though its principal diameter is trans- 

 verse and there can have been no power of rotation upon the humerus. 

 The articular surface for the latter is very simple and slightly concave 

 antero-posteriorly ; on the inner side, this surface is sharply bent down- 

 ward so as to bear laterally against the prominent internal border of the 

 humeral trochlea. The proximal facet for the ulna is curiously small and 

 is confined to a very narrow area on the posterior face of the head near 

 the inner side, and a somewhat longer, narrow area near the outer side. 

 The amount of movement between the two fore-arm bones must have been 

 very limited. 



The shaft is long, slender, irregular and with a decided, though not 

 very strong, forward curvature ; it is antero-posteriorly compressed and its 

 principal diameter is transverse, broadening distally. The distal end is 

 moderately widened and thickened, so as to be quite massive, and exter- 

 nally, the widening is abrupt, fitting into a corresponding depression of 

 the ulna, the facet for which is a large concavity. No sulci for the extensor 

 tendons are visible on the anterior face. The carpal surface is very dis- 

 tinctly divided into two facets, for the scaphoid and lunar respectively, of 

 which that for the scaphoid is much wider and is reflected farther upward 

 upon the palmar face of the radius, while the lunar facet is deeper palmo- 

 dorsally. Each facet is composed of two parts, a dorsal concavity and a 

 palmar convexity, though the two parts form one uninterrupted surface. 



The ulna (PL XIX, figs. 2, 4) has a very long olecranon, which pro- 

 jects upward and gently backward, its posterior extension not being 

 nearly so abrupt as in Macrauchenia. The sigmoid notch is deep and 

 describes nearly a semicircle. On the inner side of the proximal half of 

 the notch, the articular surface is sharply reflected upward, so as to pre- 

 sent laterally, while on the outer side, the surface is broadly divided into 

 two parts. Distally, the facet extends forward beneath the humeral 

 trochlea and appears on the inner side of the head of the radius, when 

 viewed from the front. The proximal facets for the radius are very small, 

 a larger internal, and extremely narrow external one. 



The proximal part of the shaft is triangular and very stout, rapidly be- 

 coming laterally compressed and very narrow, but remains thick antero- 

 posteriorly and is of nearly uniform cross-section throughout its length. 



