35 



equality of its vertical to its horizontal or longitudinal extent, due to the height of 

 the coronoid process, and more especially to the depth of the dentigerous part of the 

 bone. The latter dimension relates to the interesting modification of the principle 

 of maintenance of the efficiency of the masticating machinery, as contrasted with that 

 in the great proboscidian quadrupeds with a similar diet to the Megatherium. 



The condyle of the jaw (Plate XVI. fig. 1, a) is transversely elliptical, 3 inches in 

 the long, 1 inch 10 lines in the short, diameter: it is moderately convex, and least so 

 from behind forwards : it seems but a small surface for the articulation of so massive 

 a bone, laden with large teeth, to the cranium ; but the adequate firmness of suspen- 

 sion was afforded by the enormous muscles which seem to have embraced every other 

 part of the ascending ramus of the mandible. The coronoid process (Plates XII. 

 and XVI. fig. 2, b) was lofty compared with its antero-posterior diameter: it is 

 mutilated in the present skull, but seems to be entire in that of the skeleton at 

 Madrid ; and its form and extent may be appreciated in the figures published 

 by Bru * and Pander-J~. It is much compressed, begins to curve upward imme- 

 diately anterior to the neck of the condyle, being continued from the middle of that 

 part. The angular process (ib. <•) of the lower jaw curves backward 4^ inches below 

 the condyle : it is a broad triangular plate, moderately convex externally, concave 

 internally and chiefly by a slight inward bending of the lower margin, Plate XVI. 

 fig. 2, c. A few ridges on the comparatively smooth outer surface indicate the inser- 

 tions of muscles ; but the inner surface is strongly sculptured by pits and grooves 

 with strong intervening bony crests. The oblique beginning of the dentary canal 

 (e) is situated 6 inches below the condyle, and the foramen is 2 inches from the last 

 alveolus, but above its level. The anterior border of the base of the coronoid pro- 

 cess is below the interspace between the fourth and fifth alveolus ; on its inner side 

 is a large elliptical outlet of a division of the dentary canal, Plate XVI. figs. 1 & 2,/. 

 The outer and inner surfaces of the coronoid process present characters antilogous 

 to those on the same surfaces of the angular process, in regard to muscular traces, 

 but the concavity is on the outer side of the coronoid. 



The lower contour line of the mandible, which is usually continued forward, 

 straight, or with a gentle curve or undulation, in ordinary quadrupeds, is interrupted 

 in the Megatherium about one foot from the apex of the angular process by a notch, 

 from which the contour line describes an abrupt deep convex curve below the molar 

 teeth, d, and then as suddenly rises and passes by a concave curve to the under 

 side of the long and slender symphysis, Plate XII. fig. 2, d, d. The depth of the 

 dentigerous part of the horizontal ramus is 9 inches 6 lines ; it is slightly convex 

 externally, and forms a flat deep vertical wall internally, ib. d, Hi. 



* Gabkioa, J. Description del esqueletto de un quadrupedo muy corpulento y raro, &c. fol. Madrid, 

 1796, Lam. i. and ii. fig. 1. 



t Das Riesen-Faulthier (Bradypus giganteus), fol. trans., Bonn, 1821, tab. iii. See also Cuviee, ' Ossemens 

 Fossiles,' 4to, torn. v. part 1. pi. 16. figs. 1 and 2. 



