306 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 83 



remaining portion of the series the centra are about the length of 

 those in the Arabian horse; however, the length measured dorsally 

 over the anterior and posterior zygapophyses is relatively less, 

 particularly in the third, fourth, and fifth cervicals, on account of 

 the smaller size of the articular facets and less overlap of the zyga- 

 pophyses between these vertebrae than in either E. caballus or 

 E. grevyi. The width across the transverse processes and across the 

 prezygapophyses and postzygapophyses is variable but generally 

 less than in the Arabian horse at hand. 



The dorsal vertebrae (pi. 30) show marked variation in size, in 

 length and robustness of the spines, and in the development of the 

 processes but nevertheless exhibit in the articulated series a few 

 characters that appear to be significant. The length of the dorsal 

 series is distinctly longer in mature individuals than in either the 

 Arabian horse or the zebra material used to compare wdth. The 

 individual vertebrae appear to be about the size of those in the 

 Arabian horse, but the centra are relatively longer, distinctly longer 

 than in the E. grevyi column. The spines on the average are slightly 

 shorter and in several instances appear more backward directed in 

 the anterior portion of the series than in E. caballus, and after about 

 the sixth or seventh vertebrae the vertebrarterial canal is usually 

 closed posteriorly. After the first or second dorsal the ^ddth across 

 the zygapophyses is noticeably less in the fossil material, in which 

 respect the vertebrae are like those in E. grevyi. Also, the meta- 

 pophyses though variable in size and shape project forward more 

 noticeably than in the E. caballus material at hand, and in the posterior 

 part of the series are slender and elongate as in E. grevyi. The 

 posterior portion of the dorsal series though maintaining relatively 

 long centra are generally not so enlarged as in the E. caballus column. 



The relatively long centrum characterizing the dorsal vertebrae 

 is carried back in the lumbars and in most cases is noticeably slenderer 

 than in the E. caballus and E. grevrji material. The spines and trans- 

 verse processes are highly variable in length and robustness, with the 

 spines commonly elongate and moderately slender. The meta- 

 pophyses though apparently more distinct from the prezygapophyses 

 than in the E. caballus material are reduced posteriorly more notice- 

 ably than in E. grevyi. Although the individual vertebrae are 

 normally separate in the posterior portion of the series the last two 

 may be fused together (pi. 31, fig. 2), as in some recent individuals, 

 and in a few cases fused to the sacrum. 



As in the dorsal and lumbar vertebrae the segments of the sacrum 

 appear relatively elongate anteroposteriorly. The mdth is variable 

 but across the anterior portion is a little less than in the E. caballus 

 material. The distance across the lateral surfaces for articulation 



