274 



University of California Publications. 



[Geology 



A single well-preserved premolar tooth almost identical in 

 form and dimensions with the one just described was found con- 

 nected with a fragment of the jaw at another locality in the 

 Thousand Creek region. In this specimen the enamel is well 

 preserved but the tubercles are slightly worn. The cingulum is 

 well developed on the anterior side of the protocone and deutero- 

 cone, and on the outer side of the tritocone. The fragment of the 

 maxillary shows immediately above the tooth a strongly-marked 

 shoulder which formed the floor of the depression leading to the 

 infraorbital foramen as in Tayassu. The small foramina anterior 

 to the depression leading to the infraorbital foramen are immedi- 

 ately above the exposed posterior root of the tooth. Judging 

 from the position of the infraorbital foramen in Tayassu and 

 Prosthennops, unless the infraorbital foramen was here situated 

 considerably farther back than in these forms, this tooth is pos- 

 sibly P 3 rather than P 4 . 



The smaller of the two premolars is evidently more advanced 

 than the P 2 which must have occupied the very small alveolus 

 for this tooth shown in the figure of Prosthennops crassigenis 

 figured by Matthew and Gidley. 21 



The larger premolar has but three roots instead of four as 

 in P 4 of P. crassigenis, but the quadrate form is as well developed 

 as in that tooth. 



Considering the smaller premolar as either P 2 or P 3 and the 

 larger as either P 3 or P 4 , and taking all of the combinations pos- 

 sible, the Thousand Creek species is less advanced than Tayassu 

 or Mylohyus, but more advanced than Platigonus. A fully satis- 

 factory comparison of this nature with Prosthennops is not pos- 

 sible. If the smaller tooth of the Thousand Creek specimen 

 represent P 3 , Prosthennops crassigenis is apparently more ad- 

 vanced. If this tooth is P 2 , as seems possible from the situation 

 of the larger premolar with reference to the infraorbital foramen, 

 the stage of evolution of the premolars is approximately the same 

 in the two forms or slightly more advanced in the Thousand 

 Creek species. With the exception of possible differences in the 



21 Matthew, W. D., and Gidley, J. W., Bull. Amer. Mus. Nat. Hist., vol. 

 •20, p. 266, fig. 14, 1904. 



