1913] 



Merriam: Horses of Eancho La Brea 



405 



In teeth of corresponding position in the upper cheek-tooth 

 series there is considerable range in size. In general the varia- 

 tion falls within the limits which Gidley 3 has shown to hold for 

 modern Equus caballus. The variation in width of correspond- 

 ing teeth between M 2 and P 3 is commonly not more than two 

 millimeters in individuals of approximately the same age. In 

 the lower cheek-teeth there is also considerable variation in size. 



As a rule the enamel pattern of the cheek-teeth is relatively 

 simple compared with that of other described forms (figs. 7 and 

 8.) In some specimens there are noticeable variations from the 



8 



Figs. 7 and 8. Equus occidentalis Leidy. Eancho La Brea Beds, Cali- 

 fornia. Fig. 7, superior dentition, no. 21001, X Va ; fig. 8, superior denti- 

 tion, no. 12269, X %. 



normal type of this species, but in no case is the enamel sur- 

 rounding the fossettes as strongly folded as in Equus pacific us. 

 Some of the principal variations in the pattern of the cheek- 

 teeth are the following: (1) form of protoeone; (2) form of 

 post-protoconal valley; (3) position of the post-protoconal val- 

 ley; (4) plication of the anterior border of prefossette and 

 posterior border of postfossette. 



Form and size of protoeone vary considerably in the Rancho 

 La Brea specimens. The anteroposterior diameter in M 2 runs 

 from about 11 mm. in an old individual to 16 mm. in a young 



s Gidley, J. W., Bull. Amer. Mus. Nat. Hist., vol. 14, p. 102, 1901. 



