580 
PROFESSOR OWEN ON THE MEGATHERIU3I. 
and form as the second, but is somewhat narrower ; the anterior and outer angle is 
less rounded off, and the external longitudinal depression is deeper: it is further 
removed from the second tooth than this is from the first. 
The fourth molar (Plate XXIV. iv, Plate XXVI, fig. 2, iv) is smaller than the two 
preceding, but of nearly equal length, viz, 8^ inches, and is distinguished from the 
other teeth by being curved in only one direction, and that in a very slight degree, 
the concavity looking, as in the other teeth, outward : the central axis of the tooth, 
in reference to the anterior and posterior planes of the skull, is straight : the anterior 
and posterior layers of cement decrease in thickness as they approach the base of the 
tooth, so as to describe a slight curve, the convexity of which is turned, on both sides, 
towards the adjoining tooth. The fourth molar is tetragonal, and with more equal 
sides than the two preceding teeth ; the outer and inner sides are concave, the ante- 
rior and posterior ones convex ; the angles are rounded, but the anterior and inner 
angle is more produced than the rest. The grinding surface presents two equal trans- 
verse ridges, the contiguous sides of which are the longest. 
The fifth molar (Plate XXIII. fig. 1, Plate XXIV. v, Plate XXVI. fig, 2, .s) is 5 
inches in length, 1 inch 2 lines in transverse, and 10^ lines in antero-posterior diame- 
ter: its principal curvature presents its concavity forward, or toward that of the ante- 
rior tooth ; the curve in the transverse axis of the skull is scarcely appreciable. The 
transverse section of this tooth is rhomboidal, with the angles rounded, and with the 
longest diameter intersecting the antero-internal and the postero-external angles. 
The dentinal axis is transversely quadrilateral, with the posterior angles entire, and 
the posterior surface concave : the layer of cement which covers this surface is the 
thickest, and its posterior surface is convex: the layers which cover the outer and 
inner sides of the tooth are, as in the rest, the thinnest ; the anterior layer is less than 
one-third the thickness of the posterior layer. The anterior ridge of dentine is 
slightly prominent, and the posterior alone forms the summit of a transverse emi- 
nence with sloping sides, but these diverge at a more open angle than in the pre- 
ceding teeth. 
At the date of the publication of my ‘ Odontography,’ no specimen of the- lower 
jaw of the Megatherium had reached England, and certain detached teeth with slight 
differences from those known to belong to the upper jaw were conjecturally referred 
to the lower one*. The entire bone, with the dental series complete (Plate XXV., 
Plate XXI. fig. 2, i, ii, Hi, iv), shows that three of those teeth were rightly so referred; 
but that the small molar alluded to at p. 342, op. cit., does not belong to the lower 
jaw, which has only four teeth in each ramus. 
The first molar (Plate XXV. and XXI. fig. 2, z) is 8 inches in length, with a 
pulp-cavity of 5 inches in depth ; it presents a curve, with the convexity forwards, 
which is more marked than in any of the upper molars. The anterior surface is so 
much less convex transversely than in the first upper molar, that the transverse sec- 
* Odontography, p. 341. 
