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University of California Publications in Geology [ VoL - 10 



characterized by a well-marked median groove. On the occlusal surface 

 there is no distinct valley formed, but the anterior edge is worn much 

 lower than the posterior. This tooth differs decidedly from M- in 

 Me gal onyx jeffersoni, which Leidy 30 describes as follows: "The last 

 molar is a smaller tooth than the preceding pair, but has nearly the 

 same form in a reversed position, the base of the triangular section 

 ( PI. XVI, Pig. 9, a) being outward. Its posterior side is transverse 

 and is slightly concave; and the anterior side is convex and directed 

 obliquely inward. 



Inferior series. — With the loss of the first tooth in N othrotherium 

 the superior surface of the ramus directly in front of M 5 is triangular 

 in shape. The tooth-rows are relatively as far separated as in Hapalops 

 longiceps. They are very nearly parallel or diverge slightly pos- 

 teriorly. In the Pleistocene genus the characteristic features of the 

 three posterior teeth in Hapalops have been accentuated. Scott 



Fig. 8. N othrotherium shastense Sinclair. Right, inferior, dental series no. 

 16fi M. H. S. A., occlusal view, natural size. First tooth to the right is M;. 

 Rancho La Brea Beds, California. 



describes these teeth in H. longiceps as follows: ". . . 5 and 5 are 

 transverse and rectangular, and their raised margins are quite deeply 

 notched on the inner and outer sides by the transverse valley ; 5 is 

 subcylindrical, as in most of the other species." 31 



In Mo and M T{ the denser, outer layer of dentine of the tooth crown 

 is least developed along the outer and inner sides. It is perhaps less 

 developed on the outer than on the inner side. The layer is thicker 

 on the anterior and posterior faces of the crown, and appears slightly 

 better developed on the posterior face. The thickness of this more 

 compact layer of dentine determines in part the structure of the 

 occlusal surface. The outer and inner walls which are thinnest, resist 

 wear less, and are worn down more rapidly than the anterior and 

 posterior walls. The latter are therefore prominent on the wearing 



' so Leidy, J., op. cit., p. 18. 

 3i Scott, W. B., op. cit., p. 182, 1903. 



