FOSSIL HORSE REMAINS FROM IDAHO — GAZIN 311 



many specimens is relatively narrow and slightly more concave trans- 

 versely than in E. caballus or E, grevyi, although in other specimens 

 the difference is not apparent. The proportion of the distal surface 

 resting on the external splint varies appreciably but in a number of 

 specimens appears somewhat smaller than in the few unciforms of 

 Equus at hand. 



Metacarpus. — The third metacarpals (fig. 24, A) are remarkably 

 uniform in size over a large number of specimens, the extremes differ- 

 ing about 23 mm in length. The bone is near in size to that of 

 E. grevyi, but with the extremities shghtly narrower in most of the 

 specimens. The third metacarpal in E. caballus shows a far greater 

 range in size between individuals and the distal portion is relatively 

 much wider and thicker than in the fossils. Also the distal articu- 

 lating surface extends farther up the posterior surface than in the 

 fossil. 



The second and fourth metacarpals or spUnt bones appear rela- 

 tively heavy and average somewhat greater in length than in E. grevyi 

 and E. caballus. The distal extremity in many is flattened and ex- 

 panded but perhaps not enlarged to the extent seen in some recent 

 horses. 



A fifth metacarpal has been observed in only one specimen, and in 

 this case it was smaller than the trapezium that was also present. 

 The bone is nodular in shape and about 11.5 mm long, with a small 

 facet on one side. The proximal end of the fourth metacarpal in this 

 individual is relatively short anteroposteriorly, with the faceted 

 proximal surface carried a very short distance down the posterior 

 surface, presumably for the fifth metacarpal. The vestigial fifth 

 metacarpal if originally present in a larger number of individuals, 

 left no certain indication on either the fourth metacarpal or unciform 

 of its more general occurrence. It is rarely found in Equus. 



The proximal sesamoids articulating with the posterior portion of 

 the distal articulating surface are variable in size but relatively 

 narrower than in E. caballus and possibly than in E. grevyi. 



Anterior phalanges. — The first two phalanges (fig. 24, A) of the fore 

 foot, though not so much shorter than in E. caballus, are very much 

 narrower. The terminal phalanx or hoof is considerably smaller than 

 in an average horse, and the anterior surface is more steeply inclined. 

 The phalanges compare favorably with those in the zebra, although 

 the proximal end of the first phalanx is somewhat deeper antero- 

 posteriorly. 



Innominate bone.' — The pelvic girdle shows considerable variation 

 in size and proportions between individuals due in part to age difi'er- 

 ences and in a large measure to sex dimorpliism. Other than perhaps 

 a smaller average size no significant dift'erences were detected to dis- 

 tinguish the pelvis from that in modern horses. 



