486 University of California Publications in Geology [Vol.11 



and greater curvature of crown, more gradual tapering of the meso- 

 styles, and less advanced crinkling of the fossette borders. These 

 characters in Hipparion may be due to continued specialization of the 

 older Barstow form in the direction in which it was already moving. 

 So far as the cheek-tooth structure is concerned, there appears to be a 

 close relation between the Meryehippus sumani form of the Barstow 

 fauna and the Hipparion species of the Ricardo. 



Lower Check-Teeth. — The lower molars and premolars of the 

 Meryehippus forms of the Barstow fauna are somewhat longer 

 crowned than in the species of the Mascall and Virgin Valley beds, 

 and tend generally to be well cemented. They vary considerably in 

 size, corresponding to the difference between the large and small types 

 of upper molars ranging between Meryehippus intermontanus and 

 the small M. sumani type. 



In none of the Meryehippus specimens from the Barstow fauna has 

 the first lower premolar been observed though it is present in the 

 upper dentition, and is seen as a peg-like tooth in the inferior milk 

 dentition. In all of the forms of Meryehippus, the metaconid- 

 metastylid column is shorter anteroposterior^ than in Hipparion. 

 The internal groove of this column is narrow and sharp. The para- 

 stylid commonly swings inward to a plane almost even with that of 

 the inner side of the metaconid-metastylid column. Particularly in 

 the smaller forms an antero-external ridge commonly appears on the 

 protoconid. 



The small, inferior, cheek-tooth series which evidently correspond 

 to the M. sumani type of upper teeth are generally characterized by 

 somewhat shorter crowns than the large form, and by somewhat less 

 increase in the width of the series in the region of P 3 and P 4 . In the 

 small forms, the greatest width of the crushing surface of the lower 

 dentition lies at the posterior end of P 3 and anterior end of P 4 , as in 

 the large form, but the increase in width in this region seems notice- 

 ably less. In such material as it available there is a suggestion that 

 the anteroexternal angle of the protoconid appears more commonly 

 in the small form than in the large. 



In a general way, the lower cheek-teeth of Meryehippus intermon- 

 tanus (figs. 48, 49) resemble Protohippus, and might lead toward 

 Pliohippus. Those of the small M. sumani form are nearer to 

 Hipparion than is the larger type, but do not seem to approach the 

 characters of that genus as closely as do the upper teeth. 



