100 THE MAMMALIA OF THE DEEP EIVEli BEDS. 



much more deckled and abrupt. The ascending ramus appears to be shorter and the 

 condyle is less elevated above the level of the molars. 



The A r ertebral Column (PI. II, Figs. 18-20). The atlas is elongate antero-pos- 

 teriorly in proportion to its transverse width ; the anterior cotyli for the occipital 

 condyles are very deeply concave, but somewhat narrow and depressed from above 

 downward. Their lateral anterior margins are notched quite deeply and the inferior 

 portion is flared, so as to present forward instead of upward. Below, the cotyli are 

 separated only by a narrow and shallow groove, but superiorly they are kept wide 

 asunder by a very deep emargination of the neural arch, which is much more pro- 

 nounced than in A. aurelianense, Mesohippus, or ISqims. The neural spine is indicated 

 by a faintly marked ridge and is enclosed in a lyrate area formed by the surface of 

 attachment for the small posterior straight muscles of the head; this area is more 

 distinctly shown than in any other equine which I have examined. The inferior arch 

 is strongly convex and is constricted in the middle to form the deep inferior fossse ; 

 the hypapophysis is prominent and forms a large rugose tubercle. The transverse 

 processes are broken away, but enough remains to show that the atlanteo-diapophys- 

 ial notch has not been converted into a foramen ; this notch is continued backward as 

 a groove into the foramen for the first spinal nerve. The line of attachment of the 

 transverse process pursues a straight course downward and backward and does not 

 describe the slight sigmoid curve which is seen in Mesohippus. The foramen of the 

 vertebrarterial canal pierces the process on the dorsal side. The articular surfaces 

 for the axis present less directly backward than in A. aurelianense ; in shape, these 

 surfaces are triangular, with the long diameter placed vertically ; the facets are 

 reflected forward upon the inner wails of the neural canal and are connected below 

 by the broad surface for the inferior face of the odontoid process. 



Compared with the atlas of the European species, the chief difference to be 

 observed is the very much greater depth of the notch which separates the dorsal 

 margins of the anterior cotyli. 



The axis, so far as it is preserved, closely resembles that of A. aurelianense ; it 

 has a very much depressed and strongly keeled centrum, which expands anteriorly to 

 give space for the atlanteal facets. The latter are higher and narrower and rise more 

 upon the sides of the neural canal than in the horse. The odontoid process is longer 

 than in the European form and is pointed at the end, as in that species, instead of 

 being truncate, as in the horse. The articular surface on the ventral side of the pro- 

 cess is continuous with those on the centrum. The spout-like shape of the odontoid 

 is even better marked than in the existing genus, owing to the greater elevation of 

 the margins. These raised margins do not, however, extend for the full length of 



