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MEGATHERIODS.

Mylodon robustus; but the difference is less, as compared withthe lower molars in the Mylodon Darwinii. The two series ofupper molar teeth are separated by a narrower palate in theScelidothere than in the Mylodon. The first molar in the upperjaw, (PI. 80, fig. 1), is trihedral, and much less curved, and morecompressed than in the Mylodon. The second molar is alsothree-sided, and its transverse section is triangular instead ofelliptical ; and two of its sides are slightly indented : it resemblesthe antepenultimate molar in the Myl. robustus. The third andfourth molars of the Scelidothere are more compressed, and thelong axis of their transverse section is oblique, whilst in the Mylodonit is transverse to the line of the jaw. The fifth molar, besidesbeing relatively smaller, has a trihedral figure with the broadestside turned outwards, and is slightly excavated lengthwise.

In the lower jaw, the differences in the form of the teeth areof the same degree : a comparison of figure 2 with figure 5, inPlate 80, will demonstrate their nature in the Seel, leptocephalum,and Mylodon Darwinii. The lower molars of the Myl. robustus andMyl. Harlani differ still more, especially in the last species, from thoseof Scelidotherium.

The composition of the teeth closely resembles that in the genusMylodon, but the central axis of vascular dentine is relatively smaller:it is traversed, however, as in the Mylodon by medullary canals,which, at the periphery of the axis anastomose by loops, fromthe convexity of which the calcigerous tubes are given off, whichpenetrate and constitute so large a proportion of the hard dentine.This substance is about one line and a half in thickness, and isinvested by a coat of cement, not exceeding one third of a linein thickness. The teeth of the Scelidothere are thus calculated tooffer more resistance than those of the Mylodon, having a largerproportion of the hard unvascular dentine, by which they approachnearer to the structure of the teeth in the Armadillo tribe.

Megatherium. —The teeth of this most gigantic of the extinctquadrupeds of the Sloth-tribe are as small in proportion to thesize of the animal as in the Mylodon ; they are five in numberon each side of the upper jaw, and, probably, four on each side of