578 
PROFESSOR OWEN ON THE MEGATHERIUM. 
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 XXI. 
and XXV. fig. 2, h) 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 'I'. 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.r) 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 XXV. 
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 XXL fig. 2,/'. The 
outer and inner surfaces of the coronoid process present characters analogous 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 contour line, 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, and then as suddenly 
rises and passes by a concave curve to the under side of the long and slender sym- 
physis, 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 inter- 
nally, Plate XXV. fig. 2, d, Hi. 
The antero-posterior extent of the alveolar border is 9 inches (Plate XXV. fig. 1). 
The first socket is irregularly four-sided, the front side being the shortest, slightly 
convex, and with the angles rounded off between it and the outer and inner sides : 
the outer side forms rather an acute angle with the hinder side. The area of the 
second socket is tnore regularly quadrate, with the transverse diameter the longest; 
that of the third socket is nearly a true square; the fourth and last is similar to, but 
* Gaeriga, J. Descripcion del esqueletto de un quadrupedo muy coriDulento y raro, &c. fol. Madrid, 
1796, tab. i. and ii. 
t Das lliesen-Faulthier {Bradypus giganteus), fol. trans., Bonn, 1821, tab. iii. See also Cuvier, ‘ Ossemens 
Fossiles,’ 4to, tom. v. part 1. pi. 16. figs. 1 and 2. 
