470 Bulletin American Museum of Natural History. [Vol. XIX, 

 III. The Vertebral Column. 



The vertebral column of the type specimen was found com- 

 pletely articulated, even to the tip of the tail, hence the for- 

 mula can be given without a possibility of error. It is as 

 follows: Cervicals, 7; dorsals, 18; lumbars, 6; sacrals, 6; 

 caudals, 17.+ (?) i. 



The cervicals are long and slender and are horse-like in 

 general appearance. They resemble much more those .of 

 Equiis than those of MesoJiippns, but show some intermediate 

 characters. 



The atlas differs from Eqiiiis in the following characters: 

 The exterior pair of anterior foramina observed in the atlas 

 of the horse are not bridged over in Neohipparion, but are 

 represented by open notches at the anterior borders of the 

 transverse processes. The median ventral tubercle, for the 

 attachment of the longus colli muscle, is very strongly de- 

 veloped. 



The axis is comparatively longer and of less vertical thick- 

 ness than in either MesoJiippns or Eqiins. Compared with 

 Equns the odontoid process is not so deeply spout-like, and is 

 proportionately narrower. The anterior vertebral foramina 

 are comparatively large, opening directly into the side of the 

 neural canal, and are not directed forward as in Equns. In 

 Mesohippus the foramen seems not to be inclosed, but is open 

 anteriorly. The spine is divided posteriorly into two diverg- 

 ing ridges which, extending backward and downward, merge 

 into the posterior zygapophyses on either side. In MesoJiip- 

 pns the spine is not divided posteriorly, but extends back- 

 ward, ending in a strong high tubercle. 



The 3rd, 4th, and 5//? cervicals, except for their greater com- 

 parative length and more delicately formed processes, differ 

 but little from those of the modern horse. 



The ventral surface of the 6th cervical is flat, turning down- 

 ward laterally into the wing-like transverse processes, which 

 are more strongly developed than in Equns. There is only a 

 trace of the median keel, so well developed in both Mesohip- 

 pus and Eqnits. 



