AND AVES OF NORTH AMERICA. 45 



pressed, the centra much longer than deep, and deeper than wide, and with smooth con- 

 cave sides. 



The ribs of the anterior cervico-dorsal region are inserted directly in the vertically 

 oval pits of the centrum. Immediately at the point where these cease, thin transverse 

 processes appear to arise from the lower edges of the rib pits. They form a continuous 

 series with the ribs, and soon rise from the plane of the lower face of the centrum, and 

 are directed obliquely downwards. At the end of the cervical series they are directed 

 nearly vertically downwards. The number of these vertebrae is very great, the anterior 

 diminishing to a very small size, the whole measuring a little more than half the total 

 length. 



Most of the cervicals possess two venous foramina below ; the dorsals two, and most 

 of the caudals one. 



The resemblance of the caudals to the usual type of Plesiosaurus, is seen in the fact 

 that each bears near its posterior articular aspect, on the inferior face, a pair of articular 

 surfaces, for chevron bones. Similar vertebrae had been described by Leidy as the caudals 

 of a genus he called Discosaurus; the study of the present genus shows that they are 

 really of the caudals of the allied genus Cimoliasaurus. 



The ribs are simple headed ; the abdominal ribs seen in Plesiosaurus are possibly 

 wanting, as none were found by the discoverer of the fossil, after a careful search. 



The end of the muzzle, with symphysis mandibuli, was preserved. This is flat, the 

 symphysis rather short, the premaxillary grooved at the intervals between the dental alve- 

 oli. The teeth are deeply implanted, with small pulp cavity, are cylindric and furnished 

 with nearly straight elongate conic crowns, which are minutely but sharply striate to the 

 tip ; the ridges, straight, continuous. There are no indications of nostrils, so that these 

 were probably posterior and near the orbits, as in Plesiosaurus. 



The pelvic arch is more extended than the scapular, and strongly resembles the pelvic 

 arch of other Plesiosauridse. The scapular arch is peculiar ; the claviculi are broad flat 

 bones resembling the pubes of certain tortoises, while the coracoids are much like the 

 coracoids of Plesiosaurus. 



The scapular arch is remarkable for the resemblance of coracoids to those of Plesio- 

 saurus. The clavicles have a greater transverse extent than the former, and have a very 

 extensive line of union medially, and a narrow posterior prolongation which meets a simi- 

 lar anterior one of the coracoids, separating the intervening foramina. They appear to 

 form about one third of the walls of the glenoid cavity, and have a constricted base as in 

 some Plesiosauria, applied to the extremity of the coracoid. The form of the glenoid 

 cavity cannot be readily ascertained from the absence of the scapula. What we have ol 



AMEEI. PHILOSO. SOC. — VOL. XIV. 12 



