386 University of California Publications in Geology [Vol. 12 



Characters. — Relative small size, well illustrated by a comparison 

 of the referred third molar (figs. 105a--1056), with that of the referred 

 third molar of P. osborni (figs. 104a-104&). 



Description. — All small teeth, such as no. 2-4037 (fig. 106), which 

 resemble the type P. osljorni in the unusual form of the protocone, are 

 placed in this subdivision. Tooth no. 24037 (fig. 106) is believed to 

 represent P- on account of transverse proportions and the extension 

 of the anterior horn of the postfossette with respect to the posterior 

 horn of the anterior fossette. It is more markedly narrow in its pro- 

 portions than the type specimen of P. osborni (no. 23787, figs. 103a- 

 103c). This noticeable transverse narrowness is characteristic of the 

 referred smaller teeth. It is noted especially in no. 23334 (fig. 107). 

 but is seen in cross-section to be greatly influenced by the state of 

 wear, the tooth becoming thicker transversely and narrower antero- 

 posteriorly in the worn state. The lakes in moderately worn specimens 

 are small proportionally, with single folds in their opposite margins ; 

 in more worn teeth they become increasingly broader; in well worn 

 teeth they narrow and tend to disappear. 



Comparative Measurements 



P. edensis 



PUohippus P. P. osborni, (sppctans- P. edensis 



osborni francescana A sp. like) edensis 



no. 23787 no. 23275 no. 21037 no. 24039 no. 23331 

 type referred type type type 



Greatest anteroposterior diam- 

 eter 27 mm. 30.3 mm. 26 mm. 26.6 mm. 23.7 mm. 



Greatest transverse diameter .. 27 28.7 25.5 27.2 23.2 



Greatest anteroposterior length 



of metaconid-metastylid 10 8.6 8.2 7.4 8.2 



Conclusion. — The type specimen of PUohippus osborni strangely 

 approximates a P- referred to the smaller of the two horses occurring 

 in the overlying San Timoteo beds (P. francescana minor, no. 23275, 

 figs. 38a-38&; 39a-395 ; folder 3, fig. 4) . It differs widely and markedly 

 from the average PUohippus types seen in the upper teeth of its own 

 horizon, which have more of the typical PUohippus primitiveness. The 

 smaller teeth of subform A may represent no more than a sexual 

 variation. A large lower tooth, no. 23222 (fig. 127) is seen to be of 

 advanced form when compared with the average lower teeth from the 

 Eden formation, and in character and size might well belong with the 

 advanced P. osborni type represented by the upper tooth. 



