316 SCOTT. [Vol. V. 



processes for muscular and ligamentous attachments are much 

 more prominent and massive, in correlation with the increased 

 size and power of the animal. The faces of the centra are more 

 decidedly concave behind and convex in front, and are set more 

 obliquely to the long axis of the vertebrae, so that the neck 

 exhibits a more decided sigmoid curvature, as well as being 

 longer. Neural spines have disappeared from all but the 

 seventh, where the spine is much lower proportionately than in 

 Mesohippus. In Eguus asinus, however, there is a very short 

 spine on the sixth also, and the inferior lamella on this vertebra 

 is less developed than in the Miocene form. 



The number of dorsal vertebras which Mesohippus (PL XXII., 

 Figs.. 13-15) possessed is uncertain, though it can hardly have 

 been less than eighteen. The first vertebra of the series has a 

 centrum very similar in character to that of the seventh cervical, 

 but shorter, and less strongly opisthocoelous. The transverse 

 processes are quite long, and have very large, crescent-shaped 

 and deeply excavated facets for the tubercles of the first pair of 

 ribs. The prezygapophyses are very large and prominent, the 

 postzygapophyses much smaller, but projecting strongly behind 

 and to the sides of the neural spine ; the latter is stout, espe- 

 cially on the posterior edge. The succeeding dorsals gradually 

 lose these characters, the centra becoming less opisthocoelous, 

 the transverse processes shorter and with more flattened facets ; 

 the zygapophyses on the third dorsal are developed on the ante- 

 rior and posterior faces of the neural arch. Metapophyses ap- 

 pear quite early in the series and on the ninth (?) vertebra they are 

 very prominent. In the middle region the transverse processes 

 are very short and are placed above the centrum. The spines 

 are long, but very thin and much compressed ; in the anterior 

 region they have expanded tips. The posterior dorsals become 

 longer and have trihedral, sharply keeled, and slightly opistho- 

 coelous centra, with faces set obliquely to the long axis. The 

 spines are quite high, but very thin and inclined forward ; the 

 zygapophyses are more lateral in position and slightly concave 

 and convex respectively, thus being of a somewhat interlocking 

 character. 



The anterior lumbar vertebrae have elongate trihedral and 

 sharply keeled centra, which are slightly opisthocoelous and 

 have oblique faces ; in these vertebrae the transverse diameter 



