HOLLAND AND PETERSON: OSTEOLOGY OF THE CHALICOTHEROIDEA. 259 
and fusing with the body of the centrum behind the line of coéssification with the 
odontoid process, and expanding still more posteriorly, fusing with and supporting 
the lower margin of the posterior articulating surface, 
which is cup-shaped and looks upward, the posterior 
margins being strongly produced backward. At the 
points where the keel terminates behind upon the lower 
margin of the cup are formed two elevated protuber- 
ances. While a keel-like ridge on the lower side of the 
centrum is not infrequently found in the axis of mamma- 
lia, the writer does not recall any case in which this fea- 
ture is so strongly developed as it is in Moropus, though in 
the extinct genus Trigonias it is somewhat approximated. 
Among recent mammals in the genus Equus, and Antilo- 
Fig. 27. Cross-section of 
é : j axis of M. elatus (No. 1604). 
though far from being what is seen in Moropus. The 4 neural canal: BB, vertebrar- 
capra, there is a remarkable development of this ridge, 
under side of the axis of Moropus recalls the structural terial canals; CC, transverse 
features of the same bone found in some of the sauropod Processes; D, vertical keel of 
dinosaurs, with this marked difference that in the latter coptnunt eH meurae spine; 
the keel has its lower edge reinforced by a bony plate 
below, so that in cross-section it has a 1-shaped outline. The accompanying 
diagram of a vertical section through the middle of the axis (Fig. 27) shows more 
clearly than can be done by mere description the structure of the centrum and the 
sulcus on top of spine. X. 
adjacent parts. 
The lateral processes consist anteriorly of two broad lamelle, arising behind 
the inner margin of the anterior articulating face one above and the other below 
the vertebrarterial foramen, which they form by their union externally. After 
uniting to enclose the vertebrarterial canal they flare outwardly and backwardly 
in a horizontal direction, forming the long thin extremity of the lateral process, 
which projects outwardly and backwardly, and is in specimens Nos. 1713 and 
1725 thickened at the end, and is supported on the posterior portion of the centrum 
by a broad thin lamella of bone, a continuation of the lower lamina of the transverse 
process. This bony plate is deeply concave on its upper and convex on its lower 
surface, thus forming on its upper side a longitudinal groove or fossa in continu- 
ation of the lower wall of the vertebrarterial canal, the foramen of which immedi- 
ately precedes this groove. 
Between the lateral processes and the high central keel on the under side of 
the centrum are deep lateral cavities. Its outer margin and the projecting posterior 
