Apr. 1903. NorrH AMERICAN PLESIOSAURS—WILLISTON. 33 
structure is somewhat more primitive or generalized. The atlantal 
intercentrum is the largest element of the complex, save the axial 
centrum. It has five articular surfaces for union with as many bones ; 
four of these surfaces are sutural, and, doubtless in’old animals or in 
other species, the sutures may be obliterated. The inferior or ventral 
surface has an obtuse ridge along the middle, on either side of which 
the surface is flattened cr a trifle-convex. This surface is free, and 
its anterior and posterior margins are parallel. The anterior or 
cephalic surface 1s concave for articulation with the hemispherical 
occipital condyle, its rim forming more than one-third of the entire 
circumference of the cup. The posterior surface is flat, elongated 
triangular in shape, with a V-shaped emargination, for articulation 
with the axial intercentrum. Dorsally the bone articulates by a 
broad sutural surface with the odontoid, except on the cephalic part 
of each lateral margin, which unites bya small, semi-oval surface with 
the neurapophysis. | 
The axial intercentrum is not unlike the atlantal in shape, when 
seen from the ventral side, though smaller. Its ventral surface con- 
tinues the obtuse ridge of that intercentrum, but it is here quite 
prominent, the nearly square free surface on either side being dis- 
tinctly concave. The posterior surface for sutural union with the 
body of the axis is flat or gently concave; its free margin is broadly 
V-shaped, with the inferior angle rounded; the dorsal margin is 
gently concave in the middle to the truncated, very broad ars of the 
VY. The cephalic sutural surface is flat, for union with the atlantal 
intercentrum, and like that of this bone, its surface is broadly tri- 
angular in shape. On either side the bone articulates, through the 
greater part of its extent, by an oblique, concave surface with the 
axial rib, forming part of the pit for the reception of that bone. Its 
upper lateral part unites by a small surface with the odontoid, form- 
ing with it and with the axis the complete margin, of the rib-pit. 
Dorsally the bone articulates on its caudal half with the axis; on its 
cephalic half with the odontoid. 
The odontoid, or atlantal centrum, unites posteriorly by a broad, 
flattened, sutural surface with the body of the axis. On the cephalic 
side there is'a concave surface in the middle, occupying about one- 
half of the diameter for articulation with the condyle, the deep cup 
being completed ventrally by the atlantal intercentrum and on the 
sides by the neurapophyses. JDorsally the neurapophyses leave a 
small notch of the rim incomplete, which is partly filled out by the 
odontoid, making the diameter of the cupped surface of this bone 
greater dorso-ventrally than from side to side. The sides of the bone 
