FOSSIL HORSE REMAINS FROM IDAHO — GAZIN 291 



are large with a moderately simple pattern. The robust incisors are 

 noticeably erect and P^ is well developed. The dorsal and lumbar 

 vertebrae are relatively elongate with narrow centra. The zyga- 

 pophyses are comparatively close together, and the metapophyses 

 usually project forward noticeably. The limb bones are moderately 

 slender and the feet distinctly small and narrow. Greater detail will 

 be found in the comparative study of the skeleton. Many of the 

 characters there observed may be of specific importance, but com- 

 parisons were somewhat handicapped by the limited quantity of recent 

 skeletal material available, other than skulls and jaws. 



Comparisons with other species. — Of the comparisons which may be 

 made with various species of fossil horses perhaps the most pertinent 

 concerns Equus idahoensis. The species was described by Dr. J. C. 

 Merriam^" from a few teeth obtained by the University of California 

 in beds believed to belong to the Idaho formation. The type, a much- 

 worn isolated premolar, was found at a locality near Fromans Ferry, 

 about 8 miles southwest of Caldwell, Idaho. Referred to this species 

 were also an isolated lower premolar from Fromans Ferr}^ and a little 

 worn upper tooth, presumably M^, from a locality about 4)^ miles 

 southeast of Payette, Idaho, Comparisons with this species were 

 made by Dr. Gidley and accompanied his description of P. shoshon- 

 ensis, but it seems desirable here to recapitulate and add further 

 observations apparent upon examination of a large number of Hager- 

 man dentitions. 



The anteroposterior length of the type tooth of Equus idahoensis 

 is equaled or exceeded in only two Hagerman specimens, and the 

 area of the occlusal surface is greater than in any tooth, including 

 those of the largest individuals in an early adult stage. Wlien com- 

 parison is made with teeth in a comparable stage of wear the difference 

 in size is more noticeable. The development of the protocone anterior 

 to the isthmus is more progressive than in our form at this stage of 

 wear, and the constriction of the isthmus as well as the transverse 

 development of the protocone is noticeably greater. In the Hagerman 

 specimens the reentrant forming the anterior wall of the protocone 

 becomes much reduced in advanced wear, resulting in a broad union 

 between protocone and protoconule. 



The anteroposterior length of the lower premolar from Fromans 

 Ferry is equaled or exceeded in tliree of our specimens, but the width 

 and crown area are greater than in any of the Hagerman material. 

 However, the enamel pattern of this tooth is not so Equus-like as in 

 lower teeth of Equus pacificus from the Fossil Lake region of central 

 Oregon. 



The upper tooth from near Payette, referred to E. idahoensis, 

 appears less curved and is more hypsodont than in our material where 



3" Merriam, J. C, Univ. California Publ. Bull. Dept. Qeol., vol. 10, pp. 527-530, 1918. 



