GEOLOGICAL SURVEY OF THE TERRITORIES. 391 



ELASMOSAURUS, (COPE.) 



Leconte's Notes on Geology of the Route of the Union Pacific Railroad, 1808, p. G8. 

 Cope, Proceed. Acad. Nat. Sciences, 1868, p. 92. Transac. Amer. Philos. Soc, 1889, p. 44. 



This genus has been more completely preserved to us than any other 

 American representative of the order, and hence may be accepted as 

 most clearly expressive of its characters. In the interpretation of these, 

 however, considerable difficulty has been experienced, as the structure 

 form ax>pears, at first sight, to reverse, to a remarkable degree, the usual 

 proportions of known reptiles. 



The determination of the anterior extremity of the vertebral column 

 has been rendered certain by the fortunate completeness of the cervical 

 series, as the extraordinary length of the latter, equaling three times 

 that of the body, renders the most careful scrutiny necessary. 



The neural arches are every where continuous with the centra, with- 

 out sign of suture, and are externally plane. The neural canal is ex- 

 ceedingly small for the size of the vertebra?, especially on the lumbars 

 and caudals. 



The dorsal vertebra?, are remarkable from the fact that the diapophyses 

 disappear on the anterior part of the series, and gradually diminish in 

 length from behind forwards to the point of disappearance. On the 

 median and posterior parts of the series, they are very elongate, and 

 rise for a short distance from the basis of the neural arch. Anteriorly, 

 they descend and shorten, and finally remain only as the slightly ele- 

 vated borders of rib-pits. Throughout the whole of the anterior portion 

 of the column to the cervicals, the neural spines are of great elevation, 

 and of such antero-posterior extent as to be nearly continuous. 



The cervical vertebra? are not only more numerous, but become ante- 

 riorly much smaller and more attenuated than in its allies of the same 

 family. They are remarkably compressed, the centra much longer than 

 deep, and deeper than wide, and with smooth concave 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 downward. At the end of the cervical series they 

 are directed nearly vertically downward. The number of these verte- 

 bra? 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. Simi- 

 lar vertebra? 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 Cimoliasauriis. 



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

 are possibly wanting, as none are 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 alveoli. 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 



