26 MEGALONYX. 



pitted surface for junction with the acromion. The glenoid epiphysis (Fig. 3, c) 

 just mentioned, is situated between the coracoid bone and the posterior portion of 

 the glenoid articulation from which it is divided by a transverse tortuous fissure.' 

 The articular surface of the epiphysis is smooth, but that of the remainder of the 

 glenoid cavity is pitted as if it had had a thin epiphysial plate attached. 



Clavicle. — The clavicles (PI. VIII, Fig. 5), preserved in the collection of Dr. Owen, 

 are simple, much compressed, cylindroid bones, with a single curvature. They 

 present two broad surfaces ; one slightly convex in its length, the other slightly 

 concave. Of the two margins one is longitudinally convex and subacute, the other 

 concave and obtuse. The sternal extremity is the more expanded ; and it presents 

 a broad, subcircular, convex, articular head. The acromial end is flattened oval, 

 and is rough. The length of the clavicle is eight and three-quarter inches, and its 

 breadth two and a quarter inches. 



The clavicle in Dr. Dickeson's collection is rather smaller than those just described; 

 its length being half an inch less, and its breadth three-quarters of an inch less. 



Humerus. — (PI. IX, Figs. 1, 2.) The two specimens of the humerus preserved 

 in the collection of Dr. Owen, are spade-shaped bones ; and have a straight, cylin- 

 droid shaft with the distal extremity expanded into a broad trilateral plate. They 

 are relatively much shorter than in the recent sloths, but are both absolutely and 

 relatively longer than those of Mylodon robustus, and also are broader inferiorly. 



The upper two-thirds anteriorly of the diaphysis are occupied by a superficial 

 trilateral tract (Fig. 1), the base of which is formed by the outer tuberosity. This 

 tract is slightly concave longitudinally and for the greater part of its length is 

 convex transversely ; and its apex or lower end is bounded by prominent lateral 

 margins. It is neither so long, nor so conspicuous and rough as in Mylodon. Pos- 

 teriorly the upper two-thirds of the shaft are cylindroid, and present several low 

 longitudinal ridges for muscular attachment (Fig. 2). 



The head of the bone (Figs. 2, 3) is demi-oval, with the long diameter directed 

 antero-posteriorly, and measuring four inches, while the short diameter is three 

 and a quarter inches. 



The tuberosities (Figs. 2, 3) are broad, thick, and strong, are nearly equal in 

 size, and are placed upon each side of the shaft anteriorly. Their bases are con- 

 fluent in front, in which position they are separated by a shallow depression. 

 The internal tuberosity is slightly smaller and lower than the other, but in a trifling 

 degree projects more laterally. A shallow groove defines the tuberosities from the 

 head of the bone, and in its course presents several large nutritious foramina. 



The cubito-brachial articulation resembles in form one-half of an hour-glass 

 (Figs. 1, 2, 4), or it consists of a pair of convex condyles conjoined by an inter- 

 vening concave surface, which latter, together with the inner condyle, belong to the 

 ulnar articulation; but the outer condyle is rather larger and more prominent than 

 the former. 



The internal condyloid process is very much more prominent than that of 

 Mylodon ; and it is in the form of a thick pyramidal plate, with a tuberous apex, 

 which projects more than three inches beyond the line of the inner condyle. Its 

 lower margin is oblique, and is over four inches in length, and at the apex of the 



