484 University of California Publications in Geology [ V <> L - 11 



form in the relative simplicity of enamel folds of the walls bordering 

 the fossettes. 31. sumani resembles typical M. calamarius more closely 

 in the folding of the enamel, and possibly in the relative length of the 

 crown. It is considerablj' smaller than the typical form and the 

 crowns seem narrower. The apparent difference in width may be due 

 in part to difference in stages of wear of specimens compared, or to 

 method of measurement. The teeth of Cope's type are apparently 

 not in a very advanced stage of wear, but are uncommonly wide. If 

 the stage of attrition is more advanced than the writer has assumed, 

 the complication of the enamel folds becomes a relatively more im- 

 portant character. 



The large Merychippus intermontanus closely approaches the M. c. 

 stylodontus form through the medium of specimens like no. 21409 

 (fig. 40), in which the protocone unites with the protoconule in 

 incipient wear and the fossettes have relatively simple borders. The 

 crowns are, however, much larger in M. intermontanus. 



While it is possible that additions to the material from the Barstow 

 area now available may give sharper definition to boundary lines 

 between the groups of Merychippus forms here described, or possibly 

 between groups otherwise organized, it is also possible that fuller 

 collections may serve rather to make separation more difficult. This 

 may be true at least of structure of the cheek-teeth. In the series of 

 Merychippus forms of the Barstow fauna as now known, the range 

 of characters very nearly includes the range from Merychippus to 

 Protohippus and to Hipparion. In the large M. intermontanus there 

 is no clear separation from Protohippus on characters that have been 

 used up to the present time. The cheek-teeth are large, long, heavily 

 cemented, with heavy styles, comparatively simple fossettes, and the 

 protocone unites with the protoconule in incipient wear. In the 

 temporary dentition of a large specimen apparently representing 

 M. intermontanus the milk molars are long and well cemented. In 

 the 31. c. stylodontus type the relatively short crowned character is 

 present with the discrete protocone, and possibly thinner cementation. 



In the small 31. sumani form the characters of Merychippus range 

 very near those of typical Hipparion as they appear in H. mohavense 

 of the Bicardo fauna. The moderately cemented crowns may attain 

 a length equalling twice the width. The borders of the fossettes are 

 much crinkled, the protocone is discrete almost to the base and is 

 nearly circular in cross-section. Characters separating this form from 

 the Bicardo hipparions are seen in the smaller size, relative shortness 



