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University of California Publications in Geology [Vol. 10 



larger M- and M-, and a M- greatly compressed anteroposteriorly, 

 occurs also in Nothrotherium although the distinction between Inl- 

 and M- or M- is not as marked in the former as in the latter. The 

 teeth in Nothrotherium are noticeably more complex. This is indi- 

 cated by the presence of distinct, median vertical grooves on the 

 outer faces of M-, M- and M-, and to a less extent also on the inner 

 faces. Even M-, which is much more compressed anteroposteriorly 

 than the other superior teeth, shows a distinct groove on the outer 

 face and a much more faintly developed one on the inner. Nothro- 

 therium, therefore, differs from Megalonyx in which such distinct 

 lateral grooves are lacking on these teeth. In Hapalops indiffercns the 

 vertical grooving on the outer face of M-, M- and M- has been noted by 

 Scott in some specimens. In describing the corresponding teeth, as 



Fig. 7. Nothrotherium shastense Sinclair. Right, superior, dental series no. 

 208 M. H. S. A., occlusal view, natural size. First tooth to the right is M ? . 

 Rancho La Brea Beds, California. 



well as M-, in II. elongatus, Scott 28 states that "the vertical groov- 

 ing of these teeth appears to be very capriciously present or absent." 

 Judging from the appearance of the occlusal surface, the outer com- 

 pact layer of dentine in all the superior teeth of Nothrotherium is 

 best developed on the anterior and posterior surfaces and least 

 developed on the outer and inner sides. In M-, M- and M 4 this layer 

 is slightly thicker on the outer than on the inner surface. In M- the 

 layer is thickest on the posterior face, while on the outer and inner 

 faces it is of equal thickness. 



M- is smaller than M- and M 1 . Contrasted with the latter teeth, 

 M- is relatively narrower transversely in Nothrotherium than in most 

 species of Hapalops. It is relatively longer anteroposteriorly than in 

 the latter genus. The tooth may be trapezoidal in shape, with anterior 

 and posterior faces parallel as in no. 313, or these faces may converge to 

 the outer side as in skull 208 (fig. 7). The anterior and posterior faces 

 are nearly flat and the angles are well rounded. M- thus differs from 



as Scott, W. B., op. tit., p. 218, 1903. 



