480 University of California Publications in Geology [Vol. 11 



exception they are all less progressive than the species of the Ricardo. 

 Whatever be the ultimate position of these three faunal zones with 

 reference to the palaeontologic or geologic scale in other regions, there 

 seems no doubt that the horses of the Barstow fauna represent a stage 

 intermediate in time between the Mascall Middle Miocene preceding 

 and the Ricardo following. 



Upper Cheek-Teeth. — The numerous Merychippus-like specimens 

 from the Barstow fauna, resemble Merychippus calamarius (Cope) in 

 many respects, but show a wide range of variation in the characters 

 of the upper cheek-teeth. These teeth vary between small forms with 

 discrete protocone and complex enamel folds bordering the fossettes 

 on the one hand ; and large, long-crowned forms with wide, simple 

 fossettes, and protocone connected with the protoconule. The largest 

 specimens are somewhat larger than the type of Merychippus 

 calamarius, the smaller ones are considerably smaller. The difference 

 in structure between the large and small forms may amount to more 

 than specific distinction as ordinarily interpreted. It is comparable 

 in general to the difference between the genera Merychippus and 

 Protohippus, but the gradations between the forms are such that with 

 the available material the writer finds it difficult to separate distinct 

 groups of more than specific rank. 



The largest form is separated as a species distinguished from 

 Merychippus calamarius and approaching Protohippus. It has been 

 described as Merychippus {Protohippus) intermontanus 31 (figs. 36 

 to 40). This species is characterized by relatively large size; long- 

 tooth crowns ; curved, heavily cemented upper cheek-teeth with proto- 

 cone uniting with protoconule, and with relatively simple enamel 

 walls bordering the fossettes. 



A small type has been set off as Merychippus sumani. 32 It is dis- 

 tinguished (figs. 41 and 42) by relatively small size and curved, well 

 cemented, upper cheek-teeth with protocone tending to circular cross- 

 section even in advanced stage of wear, and with relatively compli- 

 cated enamel folds bordering the fossettes. 



After separation of M. sumani and the large species included in 

 Merychippus intermontanus there remains a type to some extent 

 intermediate between these two and closely approaching typical M. 

 calamarius (Cope), though the specimens seem rarely if ever to cor- 

 respond exactly to the characters of that species as shown in Cope's 



si Merriam, J. C, Univ. Calif. Publ., Bull. Dept. Geol., vol. 9, pp. 50-51, 1915. 

 32 Ibid., pp. 49-50, fig. 1, 1915. 



