Apr. 1903. North American Plesiosaurs — Willistom. 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 mmy bones ; 

 four of these surfaces are sutural, and, doubtless in old animals or in 

 other species, the sutures maybe 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 is 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 by a 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 arms of the 

 V. 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. Dorsally 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 



