222 S. W. Williston — North American Plesiosaurs. 



tions of his notes have been anticipated by myself or others ; 

 others new to me I shall fully acknowledge in each case. The 

 present paper will deal chiefly with this material, that especi- 

 ally belonging to the genus JElasmosaurus. Other material in 

 the collection will be discussed in later papers, so far as the 

 more important characters are concerned. 



Cimoliasaurus. 



The genus Cimoliasaurus has been, and yet is, poorly 

 understood. Lydekker subordinated a dozen or more generic 

 names as synonyms, some of which have been accepted as such 

 by later writers. Professor Marsh was inclined, as his notes 

 show, to accept the name Cimoliasaurus in lieu of Elasmo- 

 saurus for the species now in the Yale Museum. A brief 

 review, therefore, 'of the real characters of that genus, as 

 interpreted by the light of considerable material, will not be 

 out of place here. 



The type of the genus and species Cimoliasaurus magnus 

 Leidy is a number of dorsal and cervical vertebrae from Mon- 

 mouth county, New Jersey, probably from rocks of an epoch 

 corresponding with the Fort Pierre Cretaceous, and they have 

 been, for the most part, well figured by Leidy in his " Cretaceous 

 Reptiles." With the original specimens he later associated a 

 series of fourteen vertebrae, or rather centra of vertebrae, from 

 the same locality, and he speaks of such bones being common 

 in the deposits of New Jersey. Leidy, however, sadly misin- 

 terpreted the positions of his centra in the vertebral column, 

 nor was Cope much more correct in his interpretation of them. 

 I would interpret figures 13-15 of Plate V of the above- 

 mentioned work as of a posterior cervical centrum ; figure 16, 

 a more posterior cervical or early pectoral ; figures 17-19, a 

 median or postero-median cervical. Figures 1-3 of Plate YI 

 represent a dorsal centrum ; figure 1 is of an anterior dorsal, 

 as is also figure 5 ; figures 6 and 7 are of a posterior cervical ; 

 8-15, of median cervicals ; 16-19, of an anterior cervical. 



Leidy's description of the genus Discosaurus appeared on 

 the page following that of Cimoliasaurus, and was based 

 upon two caudal vertebrae from the Cretaceous of New Jersey 

 (Plate Y, figures 1-3) and an anterior caudal vertebra from 

 the same region. Other vertebrae from the Cretaceous of 

 Mississippi (figures 10, 11) he afterwards separated as the type 

 of a distinct species, and was probably correct in so doing. 

 Cope long ago showed the similitude of these vertebrae to 

 those of Cimoliasaurus and made the name Discosaurus a 

 synonym, in which Leidy acquiesced. 



We may therefore assume that all these vertebrae, save those 

 from Mississippi, pertain not only to Cimoliasaurus, but to 



