1917] 



Stock: Skull and Dentition of Nothrotherium 



155 



surface as two transverse ridges with intervening valley excavated in 

 the softer inner material of the crown. The type of wearing surface 

 thus obtained resembles very much that in the larger teeth of Mega- 

 therium, but in that genus the outer compact dentine is thinner and 

 the cement (on the anterior and posterior faces) relatively much 

 thicker. In the Rancho La Brea Nothrotherium the cement is more 

 evenly distributed on the crowns of the teeth, with a moderate increase 

 on the anterior and posterior faces in some specimens. In Megatherium 

 there is an excessive thickening of the cement on the anterior and 

 posterior faces. 



The complete length of the crowns of Mg, Mg and M 5 is exposed 

 in the somewhat broken specimen, no. 418. The teeth are seen to 

 reach the inferior wall of the ramus, which is very thin. In this 

 ramus the inferior teeth diverge upward in their course from the base 

 to the alveolar border. In longitudinal extent the crown of is 

 slightly concave posteriorly ; M s is practically straight posteriorly but 

 slightly concave laterally ; M 2 evidently sloped posteriorly. M 5 and 

 especially exhibit then a different longitudinal curvature from that 

 shown by the third and fourth superior teeth. 



In (fig. 8) the transverse width of the anterior half is distinctly 

 less than that of the posterior half. The transverse axis of the tooth 

 is oblique to the long axis of the tooth-row. This tooth differs from 

 the corresponding one in Hapalops and Megalonyx in the presence of 

 median vertical grooves on the inner and outer faces. The anterior 

 face is broadly concave while the posterior face is correspondingly 

 convex. On the occlusal surface the anterior transverse ridge is nearly 

 straight and may be beveled in front, while the posterior is crescentic 

 and is beveled behind. 



is slightly larger than and narrows more toward the inner 

 side. It is transversely placed with reference to the long axis of the 

 tooth-row. In occlusal view it resembles somewhat the third and fourth 

 superior teeth, but is in reverse position. If the tooth is oriented so 

 that the posterior margin of the occlusal surface corresponds to the 

 anterior margin of the third or fourth superior tooth, the former can 

 be distinguished by rounding less gradually toward the narrow lateral 

 side. In other words, the third inferior tooth has all four angles better 

 defined than those in the third and fourth superior teeth. This serves 

 also to distinguish M2 from either M- or M-. The vertical grooves of 

 the lateral faces appear, as a rule, to be more sharply defined than in 

 Mtj, while the anterior half of the tooth is not as distinctly smaller 



