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University of California Publications in Geology [Vol. ll 



same anteroposterior diameter. The metaconid-metastylid column in 

 no. 21789 is noticeably narrower than in the later form. 



In no. 21790 (fig. 182) the characters seem much as in no. 21789, 

 but the permanent teeth are shown by wear only in the case of M t . 

 In no. 21790 the milk premolars are still in position, though almost 

 worn away. The temporary teeth show a clearly marked cement 

 deposit on the best preserved individuals. 



Pliohippus tantalus is near P. supremus, in character of upper 

 cheek-teeth, but seems to have wider fossettes with a different type of 

 enamel folds. The mesostyle of the Ricardo type appears slightly 

 heavier than that of the type specimen of P. supremus, which is a 

 tooth of approximately the same size and of similar situation in the 

 dental series. 



Upper cheek-teeth of Pliohippus from the Thousand Creek Plio- 

 cene are near the stage of evolution in this Ricardo form, but the 

 material from Thousand Creek is fragmentary, and a better represen- 

 tation of the species is needed before a thoroughly satisfactory 

 comparison can be made. 



The Pliohippus forms from the Lower Etchegoin and Jacalitos of 

 the Great Valley of California all have narrower upper cheek-teeth 

 with smaller fossettes than in P. tantalus. 



PLIOHIPPUS FAIEBANKSI Merriam 



Pliohippus fairbanlesi Merriam. Univ. Calif. Publ., Bull. Dept. Geol., vol. 9, 

 p. 55, figs. 8o-8c, 1915. 



Type specimen (fig. 185) an upper cheek-tooth, P 4 ?, no. 19789, from the Ricardo 

 Pliocene near Ricardo Post Office, California. 



Crowns of upper cheek-teeth heavily cemented and strongly curved. Protocone 

 small, nearly circular in cross-section in premolars. Anterior notch between 

 protocone and protoconule sharp. Union of metaloph and protoloph commonly 

 effected by confluence of the posterior horn of the protoconule and a crochet 

 external to it. Borders of fossettes simple. 



This species differs from Pliohippus tantalus mainly in form of 

 protocone and fossettes. The protocone is of the short, round type 

 seen in P. interpolatus, while that of P. tantalus is presumed to show 

 more compression. The fossettes in the type specimen are narrow and 

 more distinctly Innate than in P. tantalus, and do not show less 

 marked plications of their walls. 



Specimen no. 21346 (fig. 183) representing the greater part of a 

 Pliohippus lower cheek-tooth dentition from the upper portion of the 

 Ricardo shows the crowns larger and heavier than those from the 

 lower and middle portion of the section referred to P. tantalus, and 



