406 University of California Publications in Geology [Vol. 7 



specimen. In general the protocone seems absolutely longer 

 anteroposteriorly and narrower transversely in young indi- 

 viduals. It is shortest anteroposteriorly and thickest trans- 

 versely in quite old individuals. Variation in stage of wear is 

 probably in part responsible for the location of the longest 

 protocone in one tooth rather than in another of the same series. 

 The writer does not consider that variation in size or form of 

 the protocone in the Kancho La Brea horses indicates the pres- 

 ence of more than one species. 



In most specimens from Rancho La Brea the post-protoconal 

 valley ends anteriorly with an oblique truncation, the truncated 

 face being directed forward and outward. In a number of 

 cases, particularly in young individuals, the enamel bordering 

 the anterior end of the valley shows a single indentation. The 

 fold is usually near the middle of the anterior end of the valley 

 in P 3 and P 4 , but is commonly situated near the outer side of 

 the anterior end in M 1 and M 2 . In individuals of fairly ad- 

 vanced age the fold is commonly absent. In aged individuals 

 there is rarely a suggestion of the fold. 



The position of the post-protoconal valley varies considerably 

 with respect to the region of the tooth margin toward which * 

 the long axis of the valley points. In the molars the axis com- 

 monly points toward the inner or lingual half of the anterior 

 border of the tooth, in P 3 and P 4 the anterior end is usually 

 directed toward the outer half of the anterior side of the tooth. 

 The position in P 3 and P 4 is referred to as erect, that in the 

 molars as depressed. The difference in position is frequently 

 related in part to difference in form of the protocone. 



The anterior and posterior fossettes of the molars and pre- 

 molars have in general relatively simple enamel borders com- 

 pared with most Pleistocene horses of North America. There 

 is quite uniformly a single clearly defined fold in the middle of 

 the anterior side of the postfossette and one on the posterior 

 inner region of the prefossette. A few minor wrinkles may also 

 be present near the major folds just mentioned. On the an- 

 terior side of the prefossette there is often a single weak fold 

 or indentation. This fold is usually strongest on the premolars 

 and on M 1 , though it may be present on the other molars. The 



