408 University of California Publications in Geology [Vol. 7 



may be poorly developed. It is often most distinct on the pre- 

 molars. Both the anterior fold of the prefossette and the pos- 

 terior fold of the postfossette may, in rare eases, be accompanied 

 by a few minor wrinkles. 



In the upper cheek-teeth the external ribs formed by para- 

 style, mesostyle, and metastyle are very strong, but even in old 

 individuals the mesostyle of the molars shows somewhat less 

 flattening externally than in E. caballus. 



The lower cheek-teeth (figs. 5, 6, 9, 10, 11) do not differ 

 greatly from those of E. caballus. Compared with specimens 

 of the domestic horse available, the Rancho La Brea form seems 

 to have relatively narrower lower premolars, but measurements 

 of other specimens of modern horses seem to indicate that this 

 character may not show a constant difference. In the pre- 

 molars the outer enamel fold between the protoconid and hypo- 

 conid is not produced between the anteroposterior folds 

 separating the metaconid and metastylid from the protoconid 

 and hypoconid. In the molars the inner end of the outer fold 

 may -extend between the two anteroposterior folds. Particularly 

 in young specimens there may be a tendency to form a small 

 secondary fold on the posterior side of the outer fold between 

 protoconid and hypoconid. In some cases the anterior end 

 of the anteroposterior fold between metastylid and hypoconid 

 may show an indentation (fig. 6), and the enamel wall on the 

 inner side of the hypoconid may show a slight crinkling. The 

 groove between the metaconid and metastylid columns is well 

 marked but wide. In general the characters of the lower teeth 

 seem close to those of E. niobrarensis. 



The milk dentition is well shown in several specimens. In 

 the upper milk molars (fig. 13), the post-protoconal valley 

 shows a light terminal indentation in specimen 20099, in which 

 M 1 is just pushing through the jaw. In no. 1983-1, a slightly 

 older specimen, the terminal indentation of the post-protoconal 

 valley has almost disappeared. There is a single indentation 

 at the anterior end of the anterior fossette, and one at the pos- 

 terior end of the posterior fossette, in each of the cheek-teeth 

 in both no. 20099 and no. 19834. 



