80 



The glenoid cavity is as narrow and bears the same disproportion to tire head 

 of the humerus, as in the Mylodon ; a resemblance which is the more satis- 

 factory, since in the specimen of Scelidotherium the humerus was cemented by 

 the stony matrix to the scapula, in its natural position. 



There is a closer correspondence between the scapulae of the Scelidothere and 

 Mvlodon than between these and that of the Megatherium ; yet all have the 

 same essential characters in common with the scapulse of the Sloths ; the Mylo- 

 don, in the differences which we have indicated between it and the Megatherium 

 in the form of the scapula, offering the nearest resemblance to the Sloths, espe- 

 cially to the two-toed species. 



Clavicle. — The clavicles are strong subcompressed bones, extending from the 

 acromial arch to the manubrium : they are slightly twisted into a double 

 curvature. The acromial end is gently bent towards its articular cavity, and 

 terminated by a smooth, elliptical, convex surface, adapted to that cavity. 

 A tuberosity is developed from the lower surface of the bone near that ex- 

 tremity. The lower margin, near the more expanded sternal extremity, forms 

 a ridge bounding an anterior concavity on half of the clavicle : the sternal ex- 

 tremity is compressed, expanded in the direction of the manubrium, to which it 

 was attached by ligament above the articulation of the first rib. 



Clavicle compared. — The closer affinity which the Unau presents, as compared 

 with the Ai, to the Mylodon and Megatherium, is illustrated in no part of its 

 organization more strikingly than by the complete clavicles which attach the 

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

 more suddenly expanded at the sternal end than in the Mylodon. The three-toed 

 Sloth in the Hunterian Museum has a small styliform clavicular bone, half an 

 inch in length, appended to the coracoid process ; not extending beyond a third 

 of the distance between that part and the sternum. 



The clavicles of the Orycterope and Ant-eaters, which are complete, have a 

 single curvature. 



The clavicle of the Megatherium is thick in proportion to its length, its ex- 

 tremities are equally developed and alike devoid of a smooth surface for syno- 

 vial cartilage, and its double curvature is more strongly marked. The clavicle 

 of the Mylodon is thus intermediate in its form and proportions between that of 

 the Megatherium and that of the two-toed Sloth. 



