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If only the os naviculare, for example, had been preserved in the present collec- 

 tion of the bones of the Mylodon, it would have proved, by the presence of its two 

 anterior articular surfaces, that only two toes, or their rudiments, had existed, 

 in addition to the two outer ones supported by the os cuboides ; in other words, 

 that the hind-foot was tetradactyle. Hence the importance of the os naviculare 

 in determining the doubtful structure of the hind-foot of the Megatherium. 

 This bone is fortunately preserved entire in the collection of remains of that 

 species presented to the College of Surgeons by Sir Woodbine Parish. It has 

 only two smooth articular surfaces, as in the Mylodon, on its anterior part ; the 

 outer surface is large, triangular, exceeding in size either of those on the os cu- 

 boides ; the inner one is of a narrow oblong form, continuous with the outer one 

 superiorly, and receding from it as it descends along the inner or tibial side of 

 the bone, with its slightly undulated plane looking forwards and inwards. In the 

 Mylodon the corresponding surface is relatively broader and shorter ; the outer 

 surface bears the same proportional size. The posterior or astragalar surface 

 of the naviculare in the Megatherium differs from that of the Mylodon in the 

 greater convexity of its upper half ; the entire bone is relatively narrower. The 

 next point to be determined is therefore the nature of the two inner toes of the 

 tetradactyle hind-foot of the Megathere, and their amount of correspondence 

 with those of the Mylodon. 



From the size and shape of the surface for the support of the external cunei- 

 form bone in the Megatherium, it is evident that the middle toe which it sup- 

 ported was relatively as large as in the Mylodon. The very close correspondence 

 in shape as well as relative size of the external cuneiform bones in both species, 

 strongly bespeaks the correspondence of character in the toe which they respect- 

 ively supported. The anterior convex articulation of the external cuneiform in 

 the Megatherium shows that the base of the middle or third metatarsal must 

 have been four inches in vertical and three inches in transverse extent, which 

 agrees with the size of that bone figured by Bru * and Pander f in the Madrid 

 skeleton. This metatarsal bone does not exist in the College collection, but a 

 second and an ungual phalanx, very nearly equalling in size those of the median 

 digit in the fore-foot, must, in my opinion, be referred to the corresponding toe 

 of the hind-foot. The osteological structure and analogies of the fore-foot of 

 * Garriga, loc. cit. pi. 5, fig. 5. f Loc. cit. tab. 4. 



