PROFESSOR OWEN ON THE MEGATHERIUM. 
579 
smaller than, the first, with the shortest and most curved side at the back part, and 
with the antero-posterior diameter a little exceeding- the transverse one. The inter- 
vals between the alveoli are narrow and subequal. 
The rami of the jaw are blended together at the symphysis, which is of great extent, 
Plate XXI. fig. 2, Plate XXV. fig. it begins posteriorly at the fore-part of 
the mandibular convexity, opposite the second alveolus, whence the symphysis 
rapidly contracts to the shape of a scoop or spout, which is prolonged 8^ inches 
from the alveolar part, and terminates in a thick, rough, rounded and emarginate 
extremity : the canal at the upper part of this spout-like symphysis is semicylindrical, 
slightly bent down at the end, and 3 inches in diameter ; it becomes roughened by 
numerous small vascular impressions near the end, but elsewhere is smooth, and has 
obviously served for the support, during acts of protrusion and retraction, of a long 
cylindrical tongue. The margins of the canal are thick and rounded. The ^mental 
foramina,’ or anterior outlets of the dental canal (P’ate XXV. g), are two on the right 
side and three on the left, from 4 lines to 8 lines in diameter. 
The teeth are of one kind, molars, five on each side of the upper jaw (Plate XXIV. 
and Plate XXVI. fi g. 2, i, ii, Hi, iv, v), four on each side of the lower jaw (Plate XXV. 
fig. 1, Plate XXI. fig. 2, i, ii, Hi, iv), eighteen in total number. 
In the upper jaw, the first or anterior molar {i) is the second in point of size, the 
last {v) being the least. The first molar (Plate XXVI. fig. 2, i) is 8^ inches in length ; 
the pulp-cavity extends six inches from the base ; it presents two slight curvatures, 
one having the convexity turned forward, and the other inward. The transverse 
section (Plate XXIV. i) gives an irregular semicircle, with the convexity turned for- 
ward, and the flat side next the second tooth ; the angles at which this side joins the 
curve are rounded ; the outer angle is somewhat produced, and the outer side of the 
curve is flattened. The central axis of the tooth, formed by the vaso-dentine *, is 
irregularly tetragonal ; the cement is thick on the anterior and posterior surfaces, 
thill on the sides of the tooth. 
The second molar (Plate XXIV., Plate XXVI. fig. 1 & fig. 2, H) is the largest of the 
upper series ; it exceeds 9 inches in length, is of a tetragonal form, with two slight 
curvatures, as in the first molar. The posterior and broadest side is nearly flat, the 
anterior side somewhat convex, the outer and narrowest side is concave, the inner side 
is sinuous, having a median longitudinal eminence between two longitudinal concavi- 
ties. The central axis of vaso-dentine (Plate XXVI. fig. 2, v) is more compressed from 
before backwards than in the preceding tooth, and its posterior surface is concave ; 
the two transverse ridges of the grinding surface of the tooth formed by the dentine 
(ib. d, d) are nearly equal ; but the sloping side formed by the vaso-dentine is larger 
than that formed by the cement (ib. c). 
The third tooth (Plate XXIV. Hi, Plate XXVI. fig. 2, Hi) is of nearly the same size 
* See ‘ Odontography/ and Art. Teeth in ‘ Cyclopaedia of Anatomy,’ vol. iv. for the definition of the different 
dental tissues. 
4 G 
MDCCCLVI. 
