92 PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 



lunar articulates only with the magnum, the facet for which is somewhat 

 less concave palmo-dorsally than in the genus last named. — The pyramidal, 

 which is not yet definitely known in Proterotherium, is elongate proximo- 

 distally and very narrow transversely, giving to the bone a scale-like 

 shape ; the proximal end bears a simply convex facet for the ulna, and the 

 distal end a small concavity for the unciform. — Of the pisiform I have 

 seen no example in either Diadiaphorus or Proterothermm ; in Thoa- 

 therhmi it is quite elongate, very slender near the proximal end, expand- 

 ing toward the free end, which is also slightly thickened and incurved. 



In none of the specimens which I have seen is the trapezium preserved, 

 but the facet for that element upon the scaphoid demonstrates its presence 

 and also that it was somewhat smaller than ■ in Proterothermm. — The 

 trapezoid is likewise rather smaller relatively than in the latter and differs 

 in being shorter proximo-distally. — As in all the known members of the 

 family, the magnum is the largest bone in the carpus ; in form, it bears a 

 general resemblance to that of Proterothermm, but is relatively somewhat 

 lower in the proximo-distal dimension, wider transversely and shallower 

 palmo-dorsally, which is chiefly due to the smaller size of the tuberosity 

 on the palmar face. The proximal surface is almost equally divided 

 between the facets for the scaphoid and lunar; the latter hardly rises so 

 prominently above the former as it does in Proterothermm and is more 

 saddle-shaped. On the distal portion of the radial side is a small, but 

 distinct, facet for the second metacarpal. — The unciform is very small, 

 especially in the transverse diameter ; the proximal facet for the pyramidal 

 is almost a groove on account of its concavity transversely : the distal end 

 bears a small facet for the vestigial fourth metacarpal and a still smaller 

 oblique surface for the head of the third. The hook-like process, which 

 projects from the palmar face, is preserved, despite the very small size of 

 the bone. 



The metacarpus (Pis. XI, fig. 12; XIII, fig. 10) consists of a single 

 functional member, mc. Ill, and two minute vestiges, which represent the 

 proximal ends of mc. Ill and IV. Yet, notwithstanding this remarkably 

 perfect monodactylism, which is more complete than in any other known 

 mammal, the metacarpals retain their primitive connections with one 

 another and with the carpus in a surprising manner. The reduction, 

 complete and thoroughgoing as it is, is yet entirely of Kowalevsky's 

 "inadaptive" type, and shows that Kowalevsky was mistaken in suppos- 



