Troxell — Vertebrate Fossils of Rock Creek, Texas. 621 



In spite of the fact that the " speed index " generally in- 

 dicates a cursorial adaptation, it is quite probable that the 

 slenderness and elongation have gone too far in this fossil type 

 for the best speed development. 



The ungual phalanx of E. calobatus, compared to that of 

 E. scotti, is very narrow, as shown by the low ratio of 71*8 per 

 cent. Compared to E. caballus africanus, it shows low ratios 

 in the transverse dimensions, but a higher ratio in the altitude, 

 corresponding to the greater height of the animal. This hoof 



Fig. 5. 



Fig. 5. Cross sections of sacra. A, Equus calobatus, n. sp.; B, E. scotti 

 and E. caballus. One-fourth nat. size. 



bone of the new species is very much like tiie narrow hoof of 

 the modern ass. 



An additional similarity between E. calobatus and E. asintis 

 is shown in the atlas, which in its general form, the shape of 

 its wings, the internal sulcus, the ventral tubercle, the foramen 

 transversarium, and the anterior ventral groove, shows a great 

 resemblance between the two. 



E. calobatus, n. sp., shows a marked contrast to E. scotti 

 and E. caballus in the form of the sacrum. In the last two 

 species, the dorsal (d, fig. 5) and ventral (v) sacral foramina enter 

 the neural canal (n) quite independently, their confluence being 

 in a wide space or vestibule on the wall of the canal. This is due 

 to the fact that the centra and thus the canal are wider than 

 the space between the dorsal foramina ; the latter therefore 

 enter the roof of the canal. On the other hand, because of the 

 narrowness of the neural cavity in E. calobatus, the dorsal 

 foramen as well as the ventral lies outside the border of the 

 neural canal. They therefore join to form the true inter- 

 vertebral foramen (i) which in turn opens into the sacral canal. 



Certain similarities between the sacrum of E. calobatus and 

 that of E. asinuj are notable, most of which are due to the 

 coossification of the spines. In E. calobatus they form a solid 

 plate with but a single foramen, representing all of the inter- 

 arcuate spaces. Such a solid fusion of the neural spines is an 

 adaptation of nature to meet the needs of a great stress in the 



