Apr. 1903. North 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 vertebrae they are directed outward and 

 backward, with the distal extremity rounded, except in those of the 

 anterior vertebras, 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 vertebrae were found in the matrix. The arches 

 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 fossae 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 



