Royal Society. 241 



large and deep hemispherical excavation on the inner part of that 

 surface for an unusually secure interlocking of the foot to the leg. 



The bones of the tarsus are six in number in the Megatherium, 

 and the astragalus offers corresponding peculiarities with those of 

 the tibia with which it is articulated, and also remarkable modifica- 

 tions for the articulation of the naviculare and calcaneum. In the 

 calcaneum,the length and strength of the hinder prominence forming 

 the great lever for the extension of the foot, are amongst its most 

 striking characteristics. These, with those of the other bones of 

 the tarsus, are minutely detailed. There is no digit answering to 

 the great toe or 'hallux,' nor any trace of the 'os cuneiforme' for 

 that toe. The innermost of the 'ossa cuneiformia' answers to the 

 middle one, and if any rudiment of the second toe ever existed in- 

 dependently, it has coalesced with that cuneiform bone: but this 

 cannot be supposed to represent both middle and internal cuneiform 

 bones and their digits blended together, as Cuvier supposed. There 

 are no little bones missing from the inner side of the middle cunei- 

 forme, as Pander and D'Alton conjectured. The first or innermost 

 distinct metatarsal bone is that of the toe answering to the third, or 

 digitus medius, in the pentadactyle foot : it is a short thick irregular 

 wedge-shaped bone, with a large triangular concave base for the 

 * ecto-cuneiforme'; a semicircular flattened surface on the outer side 

 for the fourth metatarsal, and a small semi-elliptic flat surface on 

 the inner side for the 'meso-cuneiforme'. The distal end of the 

 bone presents a strong median vertical obtuse ridge, dividing two 

 vertically elongated slightly concave surfaces, to which the anchy- 

 losed proximal and middle phalanges of the strong claw-bearing digit 

 articulate. The ungual phalanx is shorter in proportion to its depth, 

 than in the digitus medius of the fore-foot, and differs in the greater 

 breadth of the upper part of the claw-sheath, and in the straighter 

 cone, or bony core, which supported the claw. The metatarsals of 

 the fourth and fifth toes are much larger than that of the third ; but 

 they support mere rudiments of digits reduced in each to two stunted 

 phalanges, which were doubtless buried like those of the outer digit 

 in the fore-foot in a kind of callous hoof. 



Having completed the description of the skeleton, which is illus- 

 trated by an extensive series of accurate and highly finished 

 drawings, the author proceeds to the comparison of the modifica- 

 tions of the osseous structure of the gigantic extinct animal with that 

 in other known existing and extinct species of the class Mammalia. 



The teeth agree in number, kind, mode of implantation and 

 growth, with those of the Sloth, and their structure is a modification 

 of that peculiar to the Sloth-tribe. All the modifications of the 

 skull relating to the act of mastication, especially the large and 

 complex malar bone, repeat the peculiarities presented by the ex- 

 isting Sloths. There are the same hemispheric depressions for the 

 hyoid bone in the Megatherium as in the Sloth. In the number of 

 cervical vertebrae the Megatherium, like the two-toed Sloth, agrees 

 with the Mammalia generally. In the accessory articular surfaces 

 afforded by the anapophyses and parapophyses of the hinder dorsal 

 and lumbar vertebrae, the Megatherium resembles the Ant-eaters 



