62 



differs from that of the Mylodon and agrees with that of the Megathe- 

 rium in having the surface for the astragalus separated into two by an 

 intervening rough tract ; it differs from both in having the cuboidal sur- 

 face separated from the astragalar surface, but is most remarkable for the 

 form of the posterior projection, which is much compressed, and expands 

 into a broad vertical plate of bone, thus resembling the form of the 

 os calcis in the Sloths more than does that of any other Megatherioid 

 animal. The outer and anterior angle supporting part of the cuboidal 

 articulation is broken off. The original specimen is described and figured 

 by Dr. Harlan as the ' os ilium' of the Megalonyx, loc. cit., p. 336, 

 pi. xvi. 



372. The os calcis of a younger Megalonyx, from Big-bone-cave, Tenessee. 



It is more fractured than the preceding specimen, but shows the same 

 modification of the anterior articular surfaces and of the posterior pro- 

 jecting part. The original is figured and described by Dr. Harlan, loc. cit., 

 p. 329, pi. xiv. figs. 23 and 24. 



373. The ungual phalanx or claw-bone of the median digit, of probably the 



hind-foot of a Megalonyx. It resembles that of the Megatherium in its 

 great depth, but is more compressed : the osseous sheath has been 

 broken away ; its inferior base forms a convex protuberance, which is 

 notched on each side, posteriorly, for the transit of the vessel which 

 nourished the enormous claw. 



374. The corresponding claw-bone, probably of the adjoining toe: it presents 



the same general characters, but is somewhat smaller, and the base of the 

 osseous sheath, here likewise broken away, is flattened. 



375. The anterior half of a claw-bone, of similar form, but intermediate in 



size between the two preceding specimens. 



The foregoing casts of the Megalonyx Jeffersonii,/rom 348 to 375 

 inclusive, were presented by Dr. Richard Harlan. 



376. The horizontal rami and symphysis of the lower jaw of a Megalonyx. 



The jaw is deeply and firmly imbedded in a matrix, consisting of quartz 



