62 



the neural arch of the same vertebra ; the third on the tubercle for articulating 

 with the transverse process. If to these be added the two surfaces at the end of 

 the ossified sternal portion, most of the ribs of the Mylodon, Scelidothere, and 

 Megatherium, present no less than five distinct joints. I am not aware that this 

 structure exists in any other Mammal, or that it has been before noticed in the 

 Megatherium ; as it likewise obtains in the Scelidothere, it is probably character- 

 istic of the Megatherioid quadrupeds generally. 



There next remains to be considered, in connexion with the true vertebrae and 

 their appendages, how far the afiinities of the extinct Mylodon and its congeners 

 may be elucidated by a comparison of the structure of the sternum. 



The sternum of the Sloths consists of a number of small, simple, subcubical, or 

 oblong bones, having their angles truncated for the terminal articulations of the 

 ossified cartilages, and thus representing the anterior processes merely of the 

 sternum of the Mylodon. In regard to the manubrium sterni, the two-toed 

 Sloth offers the closest resemblance to the Mylodon in having that process pro- 

 longed in front of the expanded part, giving articulation to the first rib. This 

 prolongation, which is not present in the Ai, relates to the complete develop- 

 ment of the clavicles in the Unau, and serves for their ligamentous attachment. 

 The manubrium sterni has a broader and shorter figure, and is generally emar- 

 ginate anteriorly in other Edentata ; but it is in a genus of this family, viz. 

 Myrmecophaga * , to which, after the Sloths, the Megatherioids offer the closest 

 affinity, that we find in the remaining sternal bones the same remarkable 

 structure which has been described in the Mylodon. The sternal bones in the 

 true Ant-eaters, coincidently with a bifurcation of the sternal end of the ribs, 

 consist of two parts, each having articular surfaces for the sternal forks which 

 are wedged into their interspaces : but the broad depressed portion of the 

 sternal bone is anterior or external, the stumpy or cylindrical part internal, or 

 towards the thoracic cavity. 



The comparison of the sternum of the Mylodon with that of other extinct 

 Edentata, is necessarily much limited, as in no other species has so large a pro- 

 portion of this series of bones been found ; and the few that have been recog- 

 nized appertain exclusively to the Megatherium. Some of these are, however, 



* Cuvier, Ossemens Fossiles, 8vo, 1836, torn. ^dii. p. 210. 



