48 



expands to the acromial end ; the anterior surface is moderately smootli and convex : 

 the posterior surface (Plate XIX. fig. 1) is rough and more flattened, and is traversed 

 obliquely by a broad ridge, which terminates outwardly, or developes the rugged upper 

 border of the acromial half of the bone. This expanded end bends downward, as the 

 sternal end bends upward. There is a large tuberosity on the outer or fore-part of the 

 acromial expansion, which terminates in an oblong convexity, adapted to the concavity 

 beneath the expanded end of the acromion. The strong aponeurotic character of the 

 periosteum of the clavicle is well shown by the linear decussating ridges on most parts 

 of the surface of the shaft. The bone is solid. 



The closer affinity of the Bradypus didactylus, as compared with the Brad, tridactylus, 

 to the Megatherium, is illustrated by the complete clavicles which attach the scapula? 

 to the sternum ; but they are straighter, relatively more slender, and more suddenly 

 expanded at the sternal end than in the Megatherium. One species or variety of 

 Three-toed Sloth has only a small styliform clavicular bone, appended to the coracoid 

 process. The clavicle in Orycteropus and Mynnecophaga is complete, but has a single 

 curvature. The clavicle of the Mylodon is intermediate, in its form and proportions, 

 between that of the Megatherium and that of the Two-toed Sloth. 



The supposed peculiarity in the articulation of the clavicle of the Megatherium with 

 the first rib instead of the sternum, which Cuvier inferred from the figures and descrip- 

 tions of the fossil animal which had been published in his time *, is shown, by the more 

 perfect specimens since received, not to exist in nature ; and the suspicion expressed by the 

 great anatomist, viz. that it might be due to some misarticulation in the Madrid skele- 

 ton, is thus proved to be well-founded. The true connexions of the sternal ends of the 

 clavicles with each other and with the manubrium sterni, are well shown in the view of 

 the skeleton in the British Museum, given in Plate XXVII. 



Humerus. — The humerus (Plate XIX. figs. 2 — 5) is remarkable for the vast expanse 

 of the lower fourth part of the bone ; but this is limited to the transverse direction ; so 

 that, viewed sideways (as in Plate I.), the humerus of the Megatherium appears to be a 

 comparatively weak and slender bone: the whole shaft, however, gives indications of 

 the force of the thick muscular masses which surrounded and operated on it. 



The head of the bone (Plate XIX. fig. 4) presents a smooth convexity of an ellip- 

 tical form, corresponding with that of the glenoid cavity of the scapula ; the long axis 

 of the ellipse is from before backward. The head rises clear above the outer and inner 

 tuberosities, neither of which are so developed as to interfere, as in ungulate quadru- 

 peds, with the free rotation of the bone. 



The rugged surface of the inner tuberosity (ib. figs. 2 and 3, a) slopes downward and 

 inward (ulnad) from the peripheral groove of the head, marking the attachment of the 



* " D'apres lea figures et lea descriptions, il paraitrait que cette clavicule s'articulerait, non pas avec le 

 sternum comme a l'ordinaire, mais avec le bas de la premiere cote qui est recourbee, et presente une concavitc" 

 pour la recevoir. Ce serai t une singularity dont je ne connais pas d'exemple." — Ossemens Fossiles, Ed. 1835, 

 8vo, torn. viii. p. 349. 



