191-9] Merriam: Tertiary Mammalian Faunas of Mohave Desert 549 



very rare form from the Barstow, otherwise all of the Ricardo forms 

 seem more advanced than those of the Barstow. 



The PUohippus species of the Ricardo resemble in structure a 

 number of early Pliocene or late Miocene forms appearing in various 

 faunas of western North America. The Hipparicm forms of the 

 Ricardo show at least as close a resemblance to the species of Asia 

 and Europe as to any in the known American faunas. 



H1PPAEION MOHAVENSE Merriam 



Figures 163 to 1706 

 Hipparion? mohavense Merriam. Univ. Calif. Publ., Bull. Dept. Geol., 

 vol. 7, p. 436, figs, la to 3b, 1913. 



Type specimen an upper premolar three, no. 19787 (fig. 167), associated with 

 M 1 and M 2 , from the Ricardo beds, in the El Paso Range, northwestern border of 

 the Mohave Desert, California. 



Upper cheek-tooth crowns nearly straight, length a little more than twice 

 the transverse diameter in unworn specimens. Protocone small, subcircular or 

 slightly compressed laterally in the type material, separate from protoconule 

 almost to the base of the crown. Enamel bordering the fossettes with numerous 

 plications which commonly show rounded rather than angular terminations. 

 Middle region of outer side of paraeone and metacone commonly flat. Mesostyle 

 narrowing very gradually beyond the base. Cement covering well developed. 



Lower cheek-teeth with metaconid-metastylid column long anteroposteriorly, 

 narrow transversely, and showing a wide longitudinal inner furrow. The small 

 fold commonly seen on the anteroexternal angle of the lower cheek-teeth of 

 Hipparion forms is present in several specimens. The enamel of the lower cheek- 

 teeth shows a tendency to form secondary plications especially on the inner side 

 of the parastylid ridge. The greatest transverse diameter of the lower cheek- 

 tooth series is seen in P 4 , and commonly at the posterior end of that tooth. The 

 crowns are all heavily cemented. 



Since the description of the type specimen the amount of material 

 representing Hipparion mohavense has been much increased through 

 the efforts of the parties working under J. P. Buwalda in December 

 1913 to May 1915, and by several other parties. One fine specimen 

 discovered by Chester Stock includes the complete upper cheek-tooth 

 dentition (no. 21320, figs. 163, 164). This specimen is evidently 

 specifically identical with the typical form. The dimensions are 

 nearly the same as those of the type, the crowns of no. 21320 being 

 relatively narrow due to less advanced wear. The plications of the 

 enamel in no. 21320 are less marked than in the type, though the latter 

 specimen is from an older individual. There is also seen in specimen 

 no. 21320 a tendency to the formation of a slightly different pattern 

 of the enamel folds at the posterointernal angle of the prefossette, and 



