PROFESSOR OWEN ON THE MEGATHERIUM. 
581 
tion presents a tetragonal rather than a semicylindrical figure ; the anterior side, 
however, being only three-fourths the breadth of the posterior one, by which the first 
lower molar may be distinguished from all the tetragonal teeth of the upper jaw. 
Both the inner and outer sides are slightly concave transversely, the posterior side is 
moderately convex. The posterior ridge has a base twice as thick as the shorter 
anterior ridge. The greatest transverse breadth of the crown is 2 inches, the great- 
est fore and aft breadth is 1 inch 7 lines. 
The second molar (Plates XXV. and XXI. fig. 2, ii) is the largest, at least the 
broadest transversely, of those of the lower jaw. It is 9 inches in length, with a 
minor curvature, convex forward. The anterior side is the broadest, being more 
extended inward than the posterior side : its transverse diameter is 2 inches 3 lines, 
the fore and aft diameter of the crown is 1 inch 10 lines: the base of the hinder 
eminence in the latter diameter exceeds that of the front eminence chiefly by the 
greater extent of dentine exposed. 
The third molar (Plates XXV. and XXI. fig. 2, in) is of the same length as the 
second, but has its two diameters more nearly equal, the transverse section being 
nearly square, the anterior division being rather the broadest transversely, and of 
equal thickness from before backwards. Both this and the preceding tooth are con- 
vex transversely before and behind, concave at the sides. 
The last lower molar (Plates XXV. and XXI. fig. 2, iv), with an equal antero- 
posterior diameter to the preceding, is shorter and narrower transversely, especially 
in regard to its posterior division, which is more rounded, or convex transversely, 
behind, than in any of the antecedent teeth. The hinder slope of the hinder ridge 
is more nearly horizontal, and those towards the middle of the tooth are less deep : 
the modification of the grinding surface of this tooth relating to the flatter surface 
of the fifth molar above, and its greater antero-posterior extent as compared with its 
breadth compensating for the absence of a fifth molar in the lower jaw. The grind- 
ing surface of the four lower molars equals that included between the anterior ridge 
of the first molar and the posterior ridge of the last molar in the upper jaw. 
Each molar has its base undivided, but excavated by a deep conical pulp-cavity 
(Plate XXVI. fig. 2, p,])), from the apex of which cavity a fissure is continued to the 
middle of the grinding surface of the tooth, where it is more conspicuous in the upper 
(Plate XXIV.) than in the lower molars. Plate XXVI. fig. 2, exhibits a longitudinal 
section of the five molars of the upper jaw, in situ. The central axis of vaso-dentine 
{v) is surrounded by a thin layer of true or hard dentine {d), and this is coated by 
cement (c, c), which is of great thickness on the fore and hind surfaces, but is thin 
where it covers the outer and inner sides of the tooth. 
As the outer layer of the vaso-dentine is first formed by the centripetal calcification 
of the pulp, the thin crust of that substance at the open base of the tooth includes a 
space equal to the vaso-dentine at the crown of the tooth : the contraction of the 
base of the tooth is due to the progressively-diminishing thickness of the cement asi 
4 G 2 
