350 



PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



The first metacarpal is characteristically like that of Mylodon, though it 

 is relatively longer and more slender. Owen's description of this bone in 

 the Pleistocene genus applies almost exactly to Nematherium ; it "pre- 

 sents a very singular and anomalous figure, in consequence of a thick and 

 short process which is sent off from the ulnar side of its base, which gives 

 it the appearance of being bent at a right angle" ('42, 90-91). This 

 process, though not so long as in Mylodon, is very conspicuous and gives 

 to this metacarpal an entirely different appearance from that of the Santa 

 Cruz genera of the Megalonychidcz and Planopsidcz. Other differences 

 which this specimen displays from the first metacarpal of Mylodon are the 

 concave instead of the plane facet for the trapezium, the longer and more 

 slender shaft, and the well developed distal trochlea, with its prominent 

 keel. The dimensions are as follows: Length, 26 mm.; proximal width, 

 1 8 mm.; distal width, 9 mm. 



A third individual (No. 15,965, text figure 38), which consists of the 

 astragalus, navicular, part of the calcaneum, and the distal end of the fibula, 



FIG. 38. 



Nematherium sp. Left astragalus, dorsal and ventral views and left calcaneum, dorsal view, x {. 



No. 15,965- 



is referred, with some hesitation, to Nematherium, chiefly because of its 

 marked difference from the two preceding families, and a certain likeness 

 to Mylodon. The distal end of the fibula is a very massive external mal- 

 leolus, much like that of Hapalops, but with a somewhat larger facet for 

 the tibia. The astragalus has a narrow, elongate form, quite similar to 

 that of the Megalonychidcz, but a marked and suggestive likeness to 

 Mylodon is seen in the trochlea; the inner condyle has a very distinct 

 tendency to assume the odontoid shape and is much less extended prox- 

 imo-distally than the external condyle. In the astragalus of Prepotherium 



