361 R. S. Lull — Pleistocene Ground Sloth 



10265 just described, and may be assigned the same number. 

 There is in addition another right humerus not more than two- 

 fifths the linear dimensions of the first. It must pertain to 

 Mylodon, as it differs from the larger bone only in the degree 

 of development of the muscular ridges. It is a less perfect 

 bone, lacking both- articular extremities, but there is no reason 

 for considering it other than an extremely youthful member of 

 the same Mylodon species as the rest. The juvenile jaw, .No. 

 10267, is only a third smaller than the largest, so that this 

 small humerus must represent a fifth individual of less than 

 half the adult stature. To it I have assigned the catalogue 

 number 10271. 



The humerus is an immensely powerful bone resembling 

 only slightly that of the tree sloth and showing the great 

 expansion, especially of the distal end, which we are led to 

 associate with fossorial habits. The shaft of the bone is mas- 

 sive, somewhat flattened in the antero-posterior diameter, but 

 with a powerful deltoid ridge which overhangs the musculo- 

 spiral groove on the postero-external aspect. The limits of 

 the deltoid area are distinctly defined, though the surface is 

 relatively less rugose than in Owen's figures of M. robust as* 

 The pronator plate terminating in the internal supracondyloid 

 has a relatively greater lateral expanse and its upper corner is 

 more hooklike than in the figured bone. The supinator plate 

 has been broken away so that its whole extent can not be 

 determined. The trochlear shows an almost hemispherical 

 radial condyle implying great freedom of rotation of the fore- 

 arm and the ulnar condyle is saddle-shaped though tending to 

 flatness toward its inner limitation. The head of the humerus 

 exhibits a beautifully rounded segment of a spheroid passing 

 through an arc of nearly 180° in the antero-posterior direction 

 and of about 90° transversely, but the curve of the latter is of 

 greater radius. The greater and lesser tuberosities are promi- 

 nent but vary in proportions compared with those of M. 

 robustus. 



I have before me for comparison casts of two humeri, both 

 of which exceed the present one in size. One of them, a 

 right humerus from the Walhammat (Willamette), a tributary 

 of the Columbia River, was originally described by H. C. Perkins 

 in 1842.f This is one of many casts received from Doctor 

 Perkins many years ago. The humerus bears upon its surface 

 the name " Orycterotherium OregonensisP% But Doctor Har- 



*0wen, P., pi. 11, fig. 1, 1842. 



fThis Journal, (1), vol. xlii, pp. 136-140. 1842. 



| Harlan, this Journal, (1), vol. xliv, p. 80, 1843. " Dr. Perkins proposed 

 to name his specimen, if it proved to be a new animal, Orycterotherium Ore- 

 gonensis. We have casts of the os humeri from both Dr. Perkins and Dr. 

 Harlan, and can discriminate no difference ; they must have belonged to the 

 same animal. Eds." 



