37 



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 XVII. 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 XV. its, Plate XVII. fig. 2, Hi) is of nearly the same size 

 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 XV. iv, Plate XVII. 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 XIV. fig. 1, Plate XV. v, Plate XVII. fig. 2, *) is 5 inches 

 in length, 1 inch 2 lines in transverse, and 10^ lines in antero-posterior diameter: 

 its principal curvature presents its concavity forward, or toward that of the anterior 

 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 

 t lie 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 XVI., 



* Odontography, p. 341. 

 F 



