1913] Merriam: Horses of Rancho La Brea 



413 



EELATIONSHIP TO PLEISTOCENE SPECIES OF WESTERN 

 NORTH AMERICA 



In comparing the Rancho La Brea horses with the known 

 Pleistocene species of America considerable difficulty is en- 

 countered, as the larger number of described forms are based 

 upon very scanty material, usually cheek-teeth alone. Only 

 three North American Pleistocene horses are known by skulls, 

 and of these only Equus scotti is represented by more than one 

 specimen. Of Equus laurentius there is one good skull, of typical 

 Equus niobrarensis one specimen with imperfect facial and 

 frontal region. It is probable that several of the American 

 species of Equus which are considered distinct run near each 

 other in skull characters, and until the approximate limits of 

 variation are known in each, it will be difficult to make certain 

 of specific distinctions. 



Until the appearance of the important papers by Gidley 4 

 on Equus scotti, and Hay 5 on Equus laurentius and Equus nio- 

 brarensis, specific separation of American Pleistocene horses was 

 based almost entirely upon characters of the cheek-teeth, and in 

 a considerable number of species but little material was known. 

 As has been shown by Gidley, horse teeth of the same stage of 

 growth may vary markedly in size and pattern, and where 

 various stages of wear are compared the range of difference is 

 wide. As first noted by Gidley, the character of size, particu- 

 larly as seen in the transverse diameter, of the cheek-teeth seems 

 the most reliable. 



"While it is doubtless true that good specific differences appear 

 in the enamel pattern of the cheek-teeth, it is certain that such 

 characters must be used with caution. The final determination 

 of the value of these characters must depend upon examination 

 of considerable series of individuals of nearly the same age. 



Relation to Equus Occident alls Leidy. — Typical horses of the 

 genus Equus have been known fossil from California in two 

 species represented by very fragmentary remains. The first 

 form described, Equus occidentalis Leidy," was based upon a 



* Gidley, J. W., Bull. Amer. Mus. Nat. Hist., vol. 13, art. 13, 1900. 

 s Hay, O. P., Proe. U. S. Nat. Mus., vol. 44 (no. 1969), 1913. 

 o Leidy, J., Proc. Acad. Nat. Sc. Philad., 1865, p. 94. 



