340 MEGATHERIOIDS. 





and that in a very slight degree, the concavity looking, as in the 

 other teeth, outwards : the central axis of the tooth is, in reference 

 to the anterior and posterior planes of the skull, quite 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 anterior and posterior ones convex, the angles 

 rounded, and the anterior and inner one more produced than the 

 rest. The grinding surface presents two equal transverse ridges, 

 the contiguous sides of which are the longest. 



The fifth molar is five inches in length, one inch two lines in trans- 

 verse, and ten lines and a half in antero-posterior diameter: its princi- 

 pal curvature presents its concavity forwards, or towards 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 anterior internal, and the posterior external angles. The den- 

 tinal 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 is convex : those 

 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. I have not been able to extricate 

 the grinding surface of this molar tooth from the hard conglomerate, 

 in which it is imbedded ; but from the disposition of the cement 

 it is obvious that the two transverse ridges must be unequal, and 

 the anterior one the smallest, if not almost obsolete. 



No distinctly recognizable part of the lower jaw of the Mega- 

 therium has yet reached England, and our knowledge of the number 

 and kind of teeth of that jaw is derived from the descriptions and 

 figures of the Madrid Megatherium. These are wanting in the 

 requisite detail, and we have seen how little they can be trusted, even 

 in relation to the number and mode of implantation of the teeth of 

 the upper jaw. Four teeth, of similar size and shape to the large 



