288 



University of California Publications. 



[Geology 



noticeably narrower transversely. These teeth may presumably 

 be referred to Sphenophalos. They probably represent the type 

 species, S. nevadanus, which was represented by individuals 

 fully as large as the living pronghorns of the Nevada region. 



From one place at locality no. 1100 a considerable number of 

 fragments of teeth were found which include a number of pieces 

 of upper molars like those referred tentatively to Sphenophalos, 

 and with these an interesting fragment representing the wall of 

 a third lower molar. This specimen (fig. 72) shows a tooth com- 

 parable in size to M 3 in Antilocapra, but differing from that 

 form in the nature of the third or posterior lobe. In Antilo- 

 capra the posterior lobe is normally sharply divided into two 

 pillars by a deep vertical groove in the outer wall. In Capro- 

 meryx this groove is apparently barely indicated near the lower 

 end of the tooth, and in Merycodus it is unrepresented. The 

 specimen from Thousand Creek is distinctly different from the 

 existing Antilocapra in that the external longitudinal groove on 

 the posterior lobe, though clearly shown, marks only a weak 

 separation compared with the sharply-marked constriction in 

 Antilocaprar"' This tooth is presumably to be referred to 



26 In one of two specimens of Antilocapra available from northern 

 Nevada there is a notable exception to the type of M :l normal in this 

 form. In this specimen (no. 8299 Univ. Calif. Mus. Vert. Zool.) the per- 

 manent molars are present, and the last milk premolar is just on the 

 point of dropping out. In both rami M : , is a three-lobed tooth, but the 

 posterior lobe shows no indication of a division into two parts by a longi- 

 tudinal external groove. The posterior lobe in this specimen is about as 

 large as the portion of the posterior lobe anterior to the external longi- 

 tudinal groove in the typical specimens. No suggestion of an external 

 longitudinal groove is present, even low down on the tooth. The writer is 

 not entirely clear as to the significance of this variation of M 3 . If this 

 character should appear in other specimens, and at the same time be 

 coupled with other variations from the normal type, it might have some 

 claim to importance as a specific distinction. In this specimen the peculiar 

 character of M 3 is accompanied by an apparent slight modification of the 

 character of the posterior side of M 3 and by a weaker development of the 

 external styles of the upper molars. The deviation of the upper molars 

 from the type of tooth shown in other specimens available may be due 

 in part to difference in degree of wear, but this factor does not seem com- 

 petent to account for the whole difference. Unless more material of the 

 same nature as this specimen comes to hand, one would hardly seem 

 justified in considering the variation shown here as more than an indi- 

 vidual abnormality. Even if classed as an individual peculiarity, it may 

 be found to have some significance in the interpretation of the history 

 of variation or differentiation in this group, but our knowledge of the 

 meaning of such irregularities in the growth of individuals is yet too 

 imperfect to give us a clue as to the interpretation of this case. 



