1919] Merriam: Tertiary Mammalian Faunas of Mohave Desert 561 



While we are as yet very far from a situation in which we can hope 

 to establish anything like a fully satisfactory time correlation between 

 the West American Tertiary formations and those of Europe or even 

 of Asia, there are reasons for believing that the Ricardo fauna rep- 

 resents a time near the beginning of the Old World Pliocene, and 

 the Barstow fauna an epoch corresponding approximately to Upper 

 Miocene. If these suggestions are approximately correct there is a 

 possibility that the Old World hipparions are derived from an 

 American stock of which the Ricardo group was a part. Or it may 

 be that the Ricardo fauna was only a surviving remnant of this stock 

 occupying a limited area on the Pacific border of the continent. The 

 great number of American forms grouped under Neohipparion may 

 be derived from an original typical Hipparion group, or they may 

 have originated independently from another branch of Merychippus. 

 The forms of the Florida Pliocene, including Hipparion plicatile, 

 may be an Atlantic survival of the original American Hipparion 

 group differing from the western forms partly because of geographic 

 separation. 



PLIOHIPPUS TANTALUS Merriam 



Protohippus? tantalus Merriam, Univ. Calif. Publ., Bull. Dept. Geol., vol. 7, 

 p. 440, figs. 4a and 46, 1913. 



Type specimen (fig. 189) an upper premolar, no. 19434, Eicardo beds, Mohave 

 Desert, California. 



Crowns of upper cheek-teeth moderately curved. Unworn crowns probably with 

 a longitudinal diameter exceeding twice the transverse measurement. Protocone 

 connected with protoconule. Anterior and posterior fossettes wide, with enamel 

 moderately folded on their adjacent borders. Mesostyle narrowing very slightly 

 above the base, apparently somewhat heavier than in Pliohippus mirabilis and 

 P. supremus. 



A complete but somewhat worn, upper cheek-tooth dentition 

 (no. 22308, fig. 179) from locality 2065 in the middle portion of the 

 Ricardo section represents' a Pliohippus form in which the protocone 

 is relatively narrower transversely than in P. fairbanksi, and the 

 fossettes are wider. While these characters may be due in a consider- 

 able part to wear, the specimen suggests P. tantalus rather than 

 P. fairbanksi. 



Two practically complete series of lower cheek-teeth (figs. 180-182) 

 from the lower half of the section at Ricardo represent a Protohippus 

 or Pliohippus form, the relationship of which to the described upper 

 cheek-teeth has not as yet been certainly determined. This species 

 is apparently distinct from a larger, heavier type tentatively deter- 



