I 



FOSSIL HORSE REMAINS FROM IDAHO — GAZIN 309 



of the radial facet on the scaphoid side of the lunar. The posterior 

 extremity of the radial facet projects upward sharply. 



The cuneiform varies appreciably in proportions and in size and 

 shape of the facets. As in other carpal elements there is very little 

 to distinguish it from the cuneiform in E. grevyi and E. cahallus. 

 The facet for the pisiform in most specimens is not so elongate as in 

 the few cuneiforms of E. cahallus at hand. 



The pisiform corresponds closely to that in E. grevyi and E. cahallus 

 and in the highly variable facets shows no differences from these 

 forms. The posterior portion is relatively thick transversely and is 

 generally less expanded dorsoventrally than in the recent material. 



The trapezium, as indicated by the faceting on the trapezoid, was 

 originally present in nearly every case. However, in the collections 

 the trapezium is present only in the fore feet that were disarticu- 

 lated in the laboratory. Their scarcity is due no 

 doubt to their small size, hence being overlooked in 

 the field. The bone is either nodular or flattened and 

 from the trapezoid is directed backward and toward 

 the outer side of the foot, presenting a convex poster- 

 omedian surface and a longitudinally concave surface figure 23.-p;es!ppiw 

 facing outward and toward the body of the carpus. skoshonensis Gi^xey. 

 Matthew ^^ has inadvertently shown the trapezium in T^.ljL^^llm, pos- 

 articulation primarily with the scaphoid in the Blanco tenor view showing 

 skeleton. The known Hagerman specimens vary from NatL-aLTze^Hag™- 

 11 to 18 mm in length, with articular facets 8 to 13 man lake beds, upper 

 mm long. None of these show facets for articulation '°*'^°®' 

 with the second metacarpal, although such articulation probably 

 occurred in many individuals. In E. cahallus the trapezium is con- 

 sidered to be present in about half the cases, and in most of these no 

 articulation occurs wdth the trapezoid. 



The trapezoids vary appreciably in size, particularly in the length 

 of the projection carrying the posterior facet for the magnum. The 

 facet for the trapezium is nearly always large and clearly indicated 

 (fig. 23). It is continuous with the large facet for the cuneiform, 

 although in many cases it is set off by a marked transverse constriction 

 of the surface. In only 3 cases out of 72 is the facet for the trapezium 

 absent. In these instances the smooth surface does not extend 

 entirely or nearly to the distal facet for the inner splint bone; how- 

 ever, it is not unlikely that a trapezium existed but perhaps did not 

 make contact with the trapezoid. In the limited Equus material at 

 hand only one pair of E. cahallus trapezoids showed distinct facets for 

 trapezia, and two of the three pairs of E. grevyi trapezoids. 



The magnum is close in size to that in the large zebra or in the 

 Arabian horse. The surface for articulation wdth the lunar is more 



ii Matthew, W. D., Quart. Rev. Biol., vol. 1, fig. 21, p. 163, 1926. 



