78 



convex towards the body of the Mylodou, there are many strong muscular 

 ridges, but all inferior in breadth to the lowest one above described. The 

 glenoid cavity is a narrow and shallow oval cavity, extended vertically or in the 

 direction of the anterior border of the scapula. Its long diameter is equal to 

 three-fourths of the corresponding diameter of the head of the humerus ; its 

 short diameter is half that of the same diameter of the head of the humerus. 

 The want of proportion of the articular cavity to the bony sphere which plays 

 upon it is thus unusually great : so that notwithstanding the close agreement of 

 the humerus with the scapula in colour, fragility, and those characters which 

 indicate the parts of the same fossil skeleton taken from the same place and 

 bed, it was not until the observed analogy of the bones of the rest of the fore- 

 limb, with the corresponding ones of the hind limb, had left no doubt of their 

 belonging to the same animal, that the scruples arising from the disproportion 

 between the glenoid cavity and the head of the humerus, were overcome. 



Scapula compared. — Cuvier, in his description of the scapula of the Megathe- 

 rium, has pointed out its exclusive resemblance to that of the Sloths, which 

 offers, in the confluence of the acromion with the coracoid, a structure unique 

 among existing Mammals ; and the same character being present in the scapula 

 of the Mylodon, renders it needless to pursue the comparison of this bone 

 further in reference to existing species. 



I may, however, remark that the Pangolins {Manis) and the true Anteaters 

 (Myrmecophaga) make the nearest approach to the characteristic structure in the 

 scapula of the Sloths and Megatherioids, having the acromion produced nearer 

 to the coracoid than in the Orycterope and Armadillos ; that they have likewise 

 the coracoid united with the anterior angle of the scapula, which is not the case 

 in the Orycterope and Armadillos ; and that in other respects the scapula of the 

 Armadillo is that which recedes furthest from the type of that bone in the My- 

 lodon or Megatherium. 



Cuvier in his comparison refers to the tridactyle Sloths {Bradypus), but the 

 scapula of the two-toed species {Cholapus) more closely resembles that of the 

 Megatherium and Mylodon, in having the angles, especially the inferior one, less 

 rounded. The characteristic union of the acromion with the coracoid takes 

 place also at an earlier period in the didactyle than in the tridactyle Sloths ; 

 and the articular surface for the clavicle is better marked ; the clavicle itself 



