1917] 



Stock: Skull and Dentition, of Nothrotheriwn 



153 



the corresponding 1 tooth in Megalonyx, which Leidy 29 has described 

 as follows: "The second upper molar in section (PI. XVI, Fig. 9, d), 

 is quadrate with rounded angles. Its inner and posterior sides are the 

 greater, and are nearly equal; and the remaining sides are also nearly 

 equal, and are planes. The inner side is slightly convex, and the 

 posterior side is nearly a plane and is directed obliquely outward." 

 On the occlusal surface the transverse valley between the anterior and 

 posterior crests opens most broadly on the inner side in no. 208. In 

 no. 313, on the other hand, where the transverse crests are parallel, the 

 valley is of nearly the same width throughout. 



M- and M- are very similar in general shape. M- may be slightly 

 larger than M-, but the corresponding measurements of these teeth 

 are usually very close. In the greater number of species of Hapalops, 

 MA is the largest of the molariform series, and this is true in general 

 for the Santa Cruz Megalonychidae. In both teeth the anterior face 

 is broadly convex and the posterior face concave. The anterior face 

 rounds gradually to the external side ; in other words, does not form 

 a distinct angle with the outer side as in other cheek-teeth. The teeth 

 do not narrow as much externally as in Megalonyx jeffersoni, in which 

 respect they are more like M. leidyi. They lack the typical quadrate 

 cross-section characteristic of the corresponding teeth of Megatherium. 

 In M- and M- the anterior crest of the occlusal surface is decidedly 

 beveled in front, and the cutting edge of this chiseled surface is usually 

 worn into a crescent. The transverse valley is worn most broadly on 

 the inner side. In both teeth the postero-internal angle of the occlusal 

 surface is subjected to greatest wear. In several loose teeth of Nothro- 

 therium from the asphalt beds, referable to M- or M-, the outer face 

 is seen to be noticeably concave in its longitudinal extent. This char- 

 acter apparently distinguishes these teeth from M3 and especially 

 from Mg. 



M- resembles in its anteroposterior compression the corresponding 

 tooth in Hapalops and in Megalonyx. It is approximately only one- 

 half as long in that direction as the remaining superior cheek-teeth of 

 Nothrotherium. The posterior face of M- in the species from the asphalt 

 beds is very broadly concave and the concavity may become quite deep 

 as in a young individual, no. 640. The nearly flat anterior face, which 

 is of less width than the posterior face, may have a median strip 

 defined by a slight groove on each side. The inner face may have a 

 faint vertical groove near the anterior border, while the outer face is 



2n Leidy, J., op. ext., p. 17. 



