314 SCOTT. [Vol. V. 



The neural arch is not very high, and is much broader in some 

 specimens than in others ; the pedicels are perforated near the an- 

 terior edge for the passage of the second pair of spinal nerves. 

 The neural spine is very peculiar, being a long, high, hatchet- 

 shaped, thin, and very much compressed plate, which terminates 

 behind in an obtuse point, projecting considerably beyond the 

 postzygapophyses and raised much above them ; altogether 

 very different in shape and appearance from the structure seen 

 in the horse. The postzygapophyses are large and very promi- 

 nent ; in some specimens they are directed nearly straight out- 

 ward, in others more obliquely outward and backward. Some- 

 times they are quite concave from side to side, but again they 

 are as strongly convex. Transverse processes are not preserved 

 in any of the specimens which I have seen. 



The axis of Anchitherium, so far as known, is, according to 

 Kowalevsky (No. 25, p. 69, PI. I., Fig. 46), entirely like that of 

 the horse ; it differs from the axis of Mesohippus in the much 

 greater outward projection of the anterior part of the centrum 

 with the atlanteal facets. A more important difference is in the 

 shape of the odontoid, which, though still relatively short, is 

 completely spout-like in shape. 



In the horse the anterior articular surfaces project outward 

 and also downward, as in some specimens of Mesohippus ; the 

 odontoid process is spout-shaped, though not so deeply concave 

 as in the ruminants ; its facet for the atlas is continuous with 

 those on the anterior face of the centrum, the medial margins of 

 which stand far in advance of the external. The neural spine 

 is relatively low but very massive, and with a thickened rugose 

 margin, which posteriorly bifurcates into two ridges, one of 

 which passes into and dies away upon the postzygapophyses ; 

 the latter form, therefore, the hindermost projection of the ver- 

 tebra. The neural canal is proportionately low and narrow. 

 The centrum is very strongly opisthoccelous, and the posterior 

 face is obliquely placed, slanting upward and forward. 



It will be observed that the numerous individual variations of 

 the axis of Mesohippus consist chiefly in approximations to the 

 condition found in the horse, now in one respect, now in 

 another. 



The third, fourth, and fifth cervical vertebrae of Mesohippus 

 (PI. XXII., Fig. 10) are very much alike ; they all have elongate 



