PATAGONIAN EXPEDITIONS! PALAEONTOLOGY. 



length and slenderness of the shaft there is a close resemblance to 

 Megalonyx, but a likeness to Mylodon is seen in the great development of 

 the deltoid ridge, which is far larger than in the former, but in spite of this 

 great development, the " musculo-spiral course" is even less distinct than 

 in Megalonyx (Leidy, '55, PI. IX, fig. 2) ; the pectoral ridge is not conspic- 

 uous. For most of its length the shaft is irregularly cylindrical, and is 

 greatly broadened and antero-posteriorly compressed distally, though 

 relatively less so than in Megalonyx, while the very prominent and mas- 

 sive internal epicondyle is larger and the external smaller than in the 

 latter, and the bony bridge over the epicondylar foramen is broader. The 

 trochlea is wider than in the last-named genus, with much larger, sphe- 

 roidal radial facet and slightly concave ulnar surface. The anconeal fossa 

 is small and shallow, as in Mylodon; in Megalonyx it is obsolete. 



In their proportions and general appearance the forearm bones most 

 resemble those of Megalonyx, though having a certain resemblance to 

 those of Megatherium ; the extremely short and massive bones of Mylo- 

 don are entirely different. The ulna (Plate XXXII, fig. 4) is moderately 

 elongate and has a short olecranon, which, however, is relatively longer 

 than in Megalonyx or Nothrotherium ; in shape it most resembles the 

 olecranon of the former genus, but has no such transverse broadening of 

 the proximal end ; the coronoid process is more prominent and the sig- 

 moid notch more nearly approximates the arc of a circle in form ; the 

 humeral facet is narrow and its internal division is much more produced 

 distally than in the two Pleistocene genera ; the facet for the head of the 

 radius is small and nearly plane. The shaft is short, laterally compressed, 

 and somewhat curved ; the outer surface is deeply grooved, while the inner 

 has only a shallow channel near the proximal end ; the distal end is con- 

 tracted and bears a small, oblique facet for the pyramidal. 



The radius (Plate XXXII, fig. 3) is shaped much as in Megatherium, 

 but with more decided antero-posterior and lateral curvature, and the 

 more oval and projecting head gives it a less abruptly truncated appear- 

 ance ; the humeral facet is quite deeply concave and the proximal surface 

 for the ulna is very narrow transversely and confined to the posterior side 

 of the head. The shaft is strongly curved and has a convex anterior and 

 concave posterior surface ; its proximal portion is of transversely oval sec- 

 tion, the breadth not greatly exceeding the thickness ; below this the shaft 

 broadens abruptly, the internal border becoming a thin edge, and con- 



