47 



The second spine, which answers to the ridge denning the upper part of the fossa for the 

 ' teres major ' muscle in Man, runs parallel with the upper spine, and then bends down to 

 terminate in the low angle (Plate XVIII. fig. 1, e), projecting from the inferior costa of the 

 scapula, about one-third of the length of that border from the glenoid cavity (g). The 

 shape of that cavityis almost an ellipse (Plate XVIII. fig. 2, g), the lower end being scarcely 

 more contracted than the upper one : both these parts of the periphery are rather more 

 produced than the lateral borders. The entire margin of this moderately deep and well- 

 defined articular cavity is thin, but convex. The inner surface of the scapula (Plate XVIII. 

 fig. 1) is divided into many shallow depressions by intermuscular ridges (i, i), having a 

 general tendency to converge from the basal border, where most begin, towards the 

 centre of the bone, though none extend so far. The inferior costa (d) is continued 

 more directly towards the inner side of the glenoid cavity by a smooth, convex rising of 

 the inner surface, which grows broader as it gradually subsides : the fore-part of the 

 lower costa (fig. 1, e) is rather a continuation of the inferior spine; between which and 

 the portion of the lower costa (d), is the wide channel, attesting the size and force of the 

 homologue of the teres major. The vigour of the enormous subscapularis muscle is 

 manifested by the intermuscular ridges (i, i). 



Compared with the scapula of the Mylodon, that of the Megatherium differs in the 

 lower position of the spine, and the consequent greater expanse of the supraspinal fossa : 

 the base is straighter and relatively longer, the inferior costa relatively shorter. The 

 ridge extending from the inferior and posterior angle forwards upon the under surface 

 of the scapula, is relatively stronger in the Megatherium, and gives to the homologue 

 of the inferior costa in the Sloth's blade-bone the character of a second spine. As such 

 ' second spine,' with a prolongation of the scapula below, characterizes that bone in the 

 Anteaters, this indication of affinity to Myrmecophaga, or rather manifestation of the 

 more general characters of the order Bruta, by the Megatherium, is not without interest. 

 The confluence of the acromion with the coracoid is peculiar to the Sloths amongst 

 existing Mammals. 



Clavicle. — Before the discovery of the Megatherium, Man was usually cited in Manuals 

 of Comparative Anatomy as the largest animal possessing a ' collar-bone ' : he is, in fact, 

 the largest existing animal so endowed. But whilst the length of the human clavicle 

 averages but 6 inches, that of the Megatherium is 15 inches. In its general shape and 

 sigmoid curve, this bone (Plate XIX. fig. 1) singularly resembles the human clavicle, 

 but is thicker in proportion to its length. No single bone would have better excused 

 . the common conclusion of the mediaeval anatomists as to the nature of large fossil bones, 

 viz. that they were those of human giants, than the collar-bone of the Megatherium. 



The sternal end is expanded, obliquely truncate, with a rough, irregular, undulating, 

 but mainly convex articular surface, much fitted for strong ligamentous attachment to 

 the manubrium sterni, and with a narrow, rather flattened facet above this surface, 

 where it abuts against the same part in its fellow, as shown at Plate XXVII. as. 



The shaft of the bone is most contracted at its sternal third part ; thence it gradually 



H 



