1919] Merriam: Tertiary Mammalian Faunas of Mohave Desert 557 



The closest affinities of Ilipparion mohavense are evidently with 

 the Old World species of the East Asiatic H. richthofeni, and the 

 European II. gracile type, rather than with the typical American 

 species grouped in Neohipparion. Even the form seen in H. plicatile 

 of Florida is not as near the Ricardo species as are the Chinese and 

 European representatives of this genus. 



According to Schlosser 41 the upper cheek-teeth of gracile are 

 commonly distinguished from those of H. richthofeni by a relatively 

 short and broad posterior valley, a smaller and more nearly circular 

 protoeone, and by the form of the small fold of the inner wall opposite 

 the protoeone. In all of these characters H. mohavense is nearer to 

 H. richthofeni than to H. gracile. 



In the original description of Ilipparion mohavense the writer 

 suggested that diagnostic characters separating it from the Chinese 

 hipparions in the upper molars were found in the form of the small 

 fold opposite the protoeone, and in the length of the molars. Addi- 

 tional material now available shows that the small enamel fold oppo- 

 site the protoeone may have approximately the same form in the two 

 species, though it probably tends to be more complex in the Chinese 

 form. The new material available for study indicates also that the 

 difference in length of the cheek-teeth is very small. Measurements of 

 the largest specimens of H. richthofeni are somewhat greater in length 

 of crown than in the Ricardo material, but the difference is slight. 

 More noticeable than contrast in length of crown is the excess in area 

 of the cross-section of the largest cheek-teeth of H. richthofeni. 



With the collections now available few if any clearly diagnostic 

 characters in form or pattern of the Ricardo teeth designated as 

 H. mohavense appear which may be considered as certainly separating 

 them from the Chinese H. richthofeni. The most suggestive differences 

 between the two are in dimensions, but this variation is inconsiderable. 

 Were the specimens examined not found so widely separated geo- 

 graphically as to make specific identity seem highly improbable, the 

 writer would be inclined to include the Ricardo forms in the H. rich- 

 thofeni group. Geographic situation and the suggested differences 

 in dimensions make it practically certain that the Ricardo species is 

 distinct from H. richthofeni, but the marvel is that the two approach 

 so closely. It should be remarked in passing over this subject that 

 some of the characters which have been used to separate II. richthofeni 

 from II. gracile seem to disappear with the study of large collections. 



4i Schlosser, M., Abh. d. Mat. Phys. Kl. Bayr. Akad. Wiss., Bd. 22, S. 84, 1906. 



