1917] Stock: Skull and Dentition of Nothrotherium 149 



contemporary, Megalonyx. In general structure of 1 mandible Nothro- 

 therium bears the same relation to Hapalops as Megalonyx does to 

 Eucholoeops. The lower jaw of an apparently adult individual, no! 

 208, of the Rancho La Brea species is approximately twice as large 

 as that in Hapalops longiceps. Although the mandible may very 

 closely approach Megalonyx in total length, yet it differs from that 

 genus in its very decided slenderness. 



In both Hapalops and Nothrotherium the predentary portion of 

 the jaw is produced anteriorly into a long beak which in the latter 

 genus is also deeply concave. The edentulous beak or spout (fig. 5) 

 is much more elongate than in Megalonyx. The anterior end may be 

 rather acutely tapering as in no. 208 or rounding as in no. 456. 



Fig. 5. Nothrotherium shastense Sinclair. Mandible, no. 166 M. H. S. A., 

 superior view, X %. Rancho La Brea Beds, California. 



As pointed out by Scott, the lower border of the ramus in Nothro- 

 therium is less sinuous than in Hapalops. In the former genus it forms 

 an even convexity which reaches its maximum development below 

 the third inferior tooth. This border is more strongly convex than 

 in Megalonyx, but decidedly less so than in Megatherium. The inferior 

 border posterior to the tooth series and reaching to the end of the 1 

 angular process is less decidedly concave in Nothrotherium than in 

 Hapalops. 



The symphyseal keel is long and sharp, but not as prominent as in 

 Megalonyx. Usually but a single mental foramen is present. In no. 

 456, however, a small opening is situated slightly above and posterior 

 to the mental foramen of the right side. The horizontal limb of the 

 ramus (fig. 6) is noticeably of less height, especially in the anterior 

 region, and less robust than in Megalonyx. With the disappearance 



