342 MEGATHERIOIDS. 



in its slight and single curvature : its anterior and posterior sides 

 are convex ; the outer and inner ones concave. 



Another molar tooth of a Megatherium is preserved with its 

 own and a portion of the adjoining socket, showing it to be the last 

 of its series, and less than half the size of the teeth which pre- 

 ceded it. It is, in fact, as small as the last molar of the upper 

 jaw ; but differs in being straighter, and in the smaller antero- 

 posterior diameter compared with the transverse diameter : the pos- 

 terior surface is flatter, and the angles dividing the four surfaces 

 are more marked. 



The fragment of the jaw connected with this tooth is insufficient 

 to determine whether it be part of the upper or lower jaw : if of the 

 upper, the tooth clearly indicates a distinct species of Megatherium 

 from that of which the upper molar series is figured in PL 83 ; if 

 of the lower jaw, then the tooth in question may be a fifth molar, and 

 the two last molars in the upper jaw will be opposed by a fourth 

 and fifth in the lower jaw, instead of by a larger and more com- 

 plicated fourth molar, as in the other known Megatherioid genera. 



Each molar tooth of the Megatherium is excavated by an 

 unusually extensive conical pulp-cavity, (PI. 83, c) from the 

 apex of which a fissure is continued to the middle concavity 

 of the grinding surface of the tooth. The central axis of vas- 

 cular dentine, (PI. 83, b, h) is surrounded by a thin layer of 

 hard or unvascular dentine, and this is coated by the cement, 

 (ib. a, a.) which is of great thickness on the anterior and pos- 

 terior surfaces ; but is thin where it covers the outer and inner 

 sides of the tooth. As the outer layer of the vascular dentine is 

 first formed by the centripetal calcification of the pulp, the thin 

 crust of that substance at the base of the tooth includes a space 

 equal to that of the vascular 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, as it approaches that part ; 

 the intervening vacancy in the socket indicating the primitive 

 thickness of the capsular basis of the cement. 



The vascular dentine (PI. 84, a, a) is traversed throughout by 

 medullary canals, measuring j^th of an inch in diameter, which are 

 continued from the pulp-cavity, and proceed, at an angle of 50°, 



