1919] Merriam: Tertiary Mammalian Faunas of Mohave Desert 567 



The upper cheek-teeth of Pliohippus coaling ensis from the Lower 

 Etehegoin on the western border of the San Joaquin Valley show 

 rather narrow fossettes, a very small, round protocone, and a weak 

 connection of protoloph and metaloph. The Etehegoin species has 

 considerably narrower and less curved teeth than the Ricardo form, 

 and is presumably specifically different. 



The crowns of upper cheek-teeth of Pliohippus from the Thousand 

 Creek Pliocene show about the same curvature as in no. 19789, and the 

 size is approximately the same. The fossettes are larger in the Thou- 

 sand Creek form, but there is not sufficient material available for a 

 fully satisfactory comparison. The protocone is not well shown in 

 the Thousand Creek specimens. 



PLIOHIPPUS, sp. A, near MIEABILIS (Leidy) 

 A single tooth, M 1 (no. 21323, fig. 188), from Ricardo represents 

 a form presumably to be assigned to Pliohippus, but not certainly 

 identical with either P. tantalus or P. fairbanksi. Compared with the 

 described forms from the Ricardo, the crown of no. 21323 is smaller 

 and narrower, the fossettes are simpler, and the mesostyle is a little 

 lighter than in P. tantalus. As this specimen is evidently M 1 con- 

 siderably worn, while the P. tantalus type may be a P 4 in a less 

 advanced stage of wear, it is possible that position, wear, and indi- 

 vidual variation may account for the difference between no. 21323 

 and the type of P. tantalus. On the other hand the smaller size 

 suggests that later collections from Ricardo should be carefully exam- 

 ined for a species smaller than P. tantalus. The form represented by 

 no. 21323 evidently possessed characters verging on those of Plio- 

 hippus m irabilis. 



Measurements op no. 21323 



M 1 , anteroposterior diameter 21.4 mm. 



M 1 , transverse diameter 21.7 



M 1 , height of portion of crown remaining, measured along mesostyle .... 3(3 



PLIOHIPPUS MILK DENTITION 

 Several milk teeth of Pliohippus are present in the collection from 

 the Ricardo. The crowns are subhypsodont and heavily cemented. 

 Even in very slightly worn specimens the small protocone is lightly 

 connected with the protoconule. The enamel bordering the fossettes 

 shows very few plications in a moderately worn tooth. These teeth 

 resemble in general the form of milk premolars in Pliohippus 

 supremus. It is not certain which of the Ricardo species of Pliohippus 

 these teeth represent. 



