328 University of California Publications in Geology [Vol. 8 



The fourth superior tooth (third molar of P. nebrascensis) agrees 

 generally with Brown's specimen and resembles in outline the fourth 

 alveolus in the Colorado skull. The tooth is usually less compressed 

 antero-posteriorly than in M. garmani, a character already pointed 

 out by Allen for the Colorado skull. In several skulls of the Rancho 

 La Brea series, notably in no. 21155, the form of tooth in M. garmani 

 is closely approached. The diameter through the middle of the tooth, 

 from the inner anterior surface to the outer posterior surface, is never 

 as small as in Allen's specimen. 



In the fifth superior tooth (fourth molar of P. nebrascensis) the 

 sulcus on the outer surface may vary in depth, causing the posterior 

 lobe of the tooth to vary in symmetry. The tooth in M. robustus may 

 also vary in shape, and is generally closer to the postpalatine notch 

 than in the Rancho La Brea skulls. In no. 21172 the tooth is very 

 similar to that in M. garmani. 



The first inferior tooth may be reniform or oval in cross-section, 

 depending on the depth of the inner sulcus. The tooth is slightly 

 curved postero-internally, and never projects as far above the jaw as 

 in Brown's specimen. In group one the triturating surface is beveled 

 anteriorly and posteriorly where it meets the first and second superior 

 teeth. In no. 21578, a lower jaw belonging to group two, the first 

 lower tooth on the right side is beveled only posteriorly, while on the 

 left side of the jaw the tooth is beveled anteriorly and posteriorly. 

 The right tooth in this specimen is also more strongly curved postero- 

 internally than the left tooth. In an individual (no. 21156) of group 

 three the first lower teeth are not worn anteriorly but are well worn 

 postero-internally. 



The second inferior tooth may closely resemble the corresponding 

 tooth in M. garmani. The triturating surface is beveled postero- 

 internally to a greater or less degree. 



In the third inferior tooth the antero-external angle appears never 

 to be quite as prominent as in M. garmani, while the postero-internal 

 angle may be stronger than in the latter species. The tooth varies 

 somewhat in length and width of the triturating surface. 



The fourth inferior tooth is never as elongated antero-posteriorly 

 as in M. garmani. The anterior lobe is always more or less oblique 

 to the long axis of the tooth-row, while the posterior lobe may some- 

 times be at right angles to it. The isthmus connecting the two lobes 

 may develop an external crochet or much smaller middle lobe which 



