556 University of California Publications in Geology [Vol. ll 



Comparative Measurements 



Hipp ar ion 

 sp. B 



21.3 mm. 



15.8 

 ai'2 



Neohipparion 

 near molle 

 No. 21781 



H. mohavense 

 No. 21320 



M 2 , anteroposterior diameter 



M 2 , transverse diameter at summit 

 M 2 , height of slightly worn crown .. 



Chanac 

 17.8 

 al2.8 

 ai0.5 



22.3 

 20.2 

 45.6 



a, approximate. 



RELATIONSHIPS OF EICAEDO HIPPAEIONS 



The affinities of the Ricardo Ilipparion species to the Old World 

 forms cannot be determined with full satisfaction on the basis of 

 dentition alone, but the relation between dentition and limb structure 

 in the Ricardo material is as yet so indefinite as to make comparisons 

 based upon dentition less dangerous than when combined with dis- 

 cussion of limbs of doubtful relationship. 



The hipparions from the Ricardo do not seem to correspond exactly 

 in specific characters to any described American species known east 

 of the Great Basin region. The nearest forms are found in the group 

 of species including Ilipparion plicatile and //. ingenuum of the 

 Florida Pliocene, II. venustum of Ashley River, South Carolina, and 

 Neohipparion occidental of the Great Plains region. Of these H. 

 v_enustum and //. ingenuum represent much smaller forms than those 

 of the Ricardo. II. plicatile is slightly smaller than II. mohavense, 

 the crowns seem somewhat shorter anteroposterior^, the fossettes are 

 smaller, and the outer faces of the paracone and metacone pillars are 

 much more distinctly concave than in the Ricardo species. On the 

 whole these three Atlantic Coast species differ more widely from the 

 Old World hipparions than do the Ricardo forms. 



Of the American species grouped by Gidley under Neohipparion, 

 N. occidentalc shows tooth dimensions approaching those of the 

 Ricardo specimens, but probably ranges to larger size than the 

 Ricardo forms. Compared with H. mohavense the protocone of N. 

 Occident ale is commonly much longer anteroposteriorly and much 

 more compressed laterally, and the enamel bordering the fossettes 

 generally shows less marked plications. The type of H. mohavense 

 callodonte from Ricardo approaches the American Neohipparion more 

 closely than does the typical material of II. mohavense. The proto- 

 cone is somewhat flatter, the enamel may show somewhat shallower 

 plication in the upper molars, and the folds may be more distinctly 

 angular. This specimen is not however referable to any previously 

 described American species, and it is near an Old World type. 



