MEGATHERIOIDS. 339 



the lower jaw : they are more closely arranged, are longer, and more 

 deeply implanted than in the smaller Megatherioids ; they present 

 a more or less tetragonal figure, and have the grinding surface tra- 

 versed by two transverse angular ridges. Plate 83 exhibits a longi- 

 tudinal section of the five molars of the upper jaw, in situ ; and 

 demonstrates the great extent of the persistent pulp-cavity ; the 

 natural length of the series is ten inches. 



The first or anterior molar is the second in point of size, 

 the last being the least, as in the Scelidothere. The first molar 

 is eight inches and a half in length ; the pulp-cavity extends five 

 inches from the base : it presents two shght curvatures, one having 

 the convexity turned forwards, and the other inwards. The trans- 

 verse section gives an irregular semicircle, with the convexity turned 

 forward, and the flat side next the second tooth ; the angles at which 

 this side joins the curve are rounded ; the outer angle is some- 

 what produced, and the outer side of the curve is flattened. The 

 central axis of the tooth, formed by the vascular dentine, is irre- 

 gularly tetragonal, the cement is thick on the anterior and posterior 

 surfaces, thin on the sides of the tooth. 



The second molar is the largest of the upper series ; it exceeds 

 nine 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 

 narrow^est side is concave, the inner side is sinuous, having a median 

 longitudinal eminence between two longitudinal concavities. The 

 jcentral axis of vascular dentine 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 are nearly equal ; but the sloping side formed by the 

 dentine, is larger than that formed by the cement. 



The third tooth is of nearly the same size and form as the 

 second ; but is somewhat narrower, the anterior or outer angle is 

 less rounded off, and the external longitudinal depression is deeper. 



The fourth molar is smaller than the two preceding, but of 

 nearly equal length, viz. eight inches and a half, and is distinguished 

 from all the other teeth, by being curved in only one direction, 



z 2 



