49 



joint-capsule. The pectoral ridge is continued from the lower, slightly outstanding, 

 part of this tuberosity. The outer tuberosity (ib. b) projects from a lower level, is larger 

 and more prominent than the inner one ; and is divided into two subequal rough facets. 

 The outer deltoid ridge begins from the outer side of the tuberosity ; the inner deltoid 

 ridge (ib. d) some inches lower down, and nearer the inner side of the shaft: these 

 ridges converge, strengthen as they descend, and coalesce at the beginning of the lower 

 third of the shaft, defining a long and narrow angular tract for the implantation of the 

 powerful deltoid muscle. A tuberosity (ib. c) is developed from the outer (radial) side 

 of the humerus, one-third down the bone, from which a strong ridge descends along the 

 same side of the middle third : this ridge is defined below, and divided from the supi- 

 nator ridge, by a deep and smooth oblique channel (ib. e) for the passage of vessels and 

 nerves from the back to the fore-part of the bone. The supinator ridge (ib. f) is the 

 most prominent feature of the lower third of the humerus ; it presents a long, triangular, 

 rough outer facet, widening as it descends, and with a secondary ridge from its middle 

 part. The longitudinal contour of this facet forms an obtuse angle with the lower half 

 of the ridge, extending to the outer condyle of the humerus : the outer facet of this half 

 is rough, triangular, with the base upward. The pectoral ridge (ib. fig. 3, i) terminates 

 in a low tuberosity (ib. h) on the inner side of the middle of the shaft ; whence a second 

 ridge is continued upward upon the back of the shaft. This surface is flatter than the 

 fore-part, especially at its lower expanded third ; at the bottom of which, midway between 

 the outer and inner supracondyloid productions, and just above the lower articular sur- 

 face, is a small but well-defined olecranal depression. The inner supracondyloid angular 

 production (ib. k) has the flat rough facet only upon its lower half. Owing to the pro- 

 duction of these ridges, the articular condyles themselves (ib. g, I) appear to occupy but 

 a small part of the distal end of the bone ; for the extreme breadth of this end being 

 1 3 inches, that of the articular surface is but 7^ inches. It consists of two convexities, 

 side by side, divided by a narrow and deep channel, continued from the front non-arti- 

 cular surface half-way towards the back part of the bone. Both convexities have a full 

 elliptic periphery ; the outer one (ib. figs. 2, 3, 5, g) with the long axis from before 

 backward, the inner one (ib. I) with the long axis from side to side : the outer condyle is 

 the larger and more prominent of the two ; it forms more than a hemisphere, the antero- 

 posterior contour describing full three-fourths of a circle. The extent of flexion and 

 extension of the fore-arm on the arm is thus shown to be considerable. The articular 

 surface continued from one condyle to the other is concave transversely. 



The centre of the shaft of the humerus is occupied throughout by a coarse cancellous 

 structure. A very small medullary artery penetrates the back part of the bone, below 

 the pectoral tuberosity, the canal extending obliquely downward and outward. 



The Myrniecophaga didactyla, amongst existing Bruta, most resembles the Megathe- 

 rium in the development of the supinator or outer supracondyloid ridge. 



In its general proportions, the humerus of the Megatherium resembles that of the 

 Megalonyx; and is more slender, in proportion to its length, than in the Mylodon or 



H 2 



