1913] 



Merriam: New Anchitheriine Horses 



431 



than Parahippus, it might be suspected that the Mohave species is 

 allied to it. 



The only known material representing the lower jaw of 

 Archaeohippus available for comparison consists of two frag- 

 mentary specimens from the Mascall Miocene, the typical horizon 

 of that genus. This material was referred to Archaeohippus 

 by Gidley. 5 The principal specimen is a piece of a lower jaw with 

 P 3 and P\ and the roots of P 1 and P 2 . The teeth present are 

 unfortunately much worn, and the nature of the cusps cannot 

 be determined. The important characters shown are the dimen- 

 sions of the premolars, and the well-developed internal basal 

 eingulum on the molariform teeth. It seems probable that 

 Gidley 's reference of the lower teeth from the Mascall to the 

 genus Archaeohippus is correct, as these teeth resemble the 

 typical upper teeth in the presence of a basal eingulum, just as 

 the lower teeth from the Mohave Beds resemble the upper teeth 

 from that region in the absence of cingula excepting at the 

 anterior and posterior ends. 



The dentition of specimen 19764 differs from the lower teeth 

 referred to Archaeohippus by Gidley in the absence of external 

 and internal cingula, and apparently also in the proportions of 

 the premolars. 



The form represented by the lower jaw, no. 19764, shows a 

 general resemblance to Hypohippus, but differs in its slightly 

 higher and more rugose crowns, more clearly marked incipient 

 separation of metaconid and metastylid columns, and absence of 

 external basal eingulum. 



The lower jaw differs from typical Parahippus in the very 

 weak separation of the metaconid and metastylid columns, and 

 in the absence of cement from the crowns. The separation of 

 metaconid and metastylid in no. 19764 shows but little advance 

 beyond the stage seen in the dentition of a Hypohippus speci- 

 men from Virgin Valley.' 1 In none of the cheek-teeth of no. 

 19764 are metaconid and metastylid pillars separated on the inner 

 side by more than a faint groove at the summit. 



■> Gidley, J. W., Bull. Amer. Mus. Nat. Hist., vol. 22, p. 385, 1906. 

 6 See Gidley, J. W., Univ. Calif. Publ. Bull. Dept. Geol., vol. 5, p. 236, 

 fig. 1, 1908. 



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