Apr. 1903. NorrTH AMERICAN PLESIOSAURS—WILLISTON. 37 
in the middle of the concavity, sometimes apparently obsolete. The 
margins are rounded, the cartilaginous borders limited exteriorly by 
a slender, smooth line. On the ventral surface, near the middle 
antero-posteriorly, there are two vascular foramina, at the bottom of 
a slight depression, and separated from each other by a rounded 
ridge. Posteriorly the distance between these foramina becomes 
greater. The pit for the articulation of the cervical rib occupies 
nearly the whole length of the centrum, between the cartilaginous 
margins, and reaches also nearly to the plane of the neural sur- 
face. The ribs increase in length and width from the axis to the 
beginning of the thorax. They are single headed, flattened, with the 
free extremity moderately dilated and thinned, except those of the 
axis, which are more styliform, and are directed more obliquely back- 
ward. On the following vertebre they are directed outward and 
backward, with the distal extremity rounded, except in those of the 
anterior vertebre, where the anterior part distally is slightly angu- 
lated, as though suggesting a rudimentary anterior projection. The 
spines slope gently backward. They increase but little in length, 
that of the third vertebra measuring forty-eight millimeters in length 
while that of the last is but fifty-five above the zygapophyses. They 
increase in stoutness, however, much more than in length, the ante- 
rior ones being slender, the posterior broad and thick with a some- 
what expanded cartilaginous extremity. The zygapophyses are 
broadly separated throughout, with an obliquity of about thirty 
degrees from the dorsoventral plane. They are broadly oval in out- 
line, of large size and nearly flat. They project strongly from the 
body of the vertebra, leaving a space of about ten millimeters in 
extent between the centra, when in close articulation, for the inter. 
vening cartilage. The diameter of the spinal canal throughout the 
series is about fifteen’millimeters. Upon the floor of the canal'there 
are two venous foramina, near the middle. 
Thirty dorsal vertebre were found in the matrix. The arches 
were invariably separated from the centra, and, because of the general 
resemblance of the latter it is impossible to say whether all have been 
correctly associated. The centra were in large part crushed or com- 
pressed, and in some cases were so soft that it was found impossible 
to remove them entire. Two of the arches were wedged into the 
temporal fosse of the skull in such a way that it was found necessary 
to largely destroy them in cleaning the skull. In general, the spines 
of the dorsal region were so soft and frail, intermingled as they were, 
that they could not be removed. A large part of them, hence, have 
been modeled in the restored skeleton. Because, however, of not a 
