

S. W. Williston — North American Plesiosaurs. 223 



the type species as well ; from which certain definite generic 

 characters are evident. Cope erred in most of the distinctive 

 characters that he assigned to the genus Elasmosaurus, but 

 was correct in an important one, — the length of the neck. 

 We shall see that in all the known species assigned to Elasmo- 

 saurus, of which this part of the skeleton is known, the neck 

 is very long, and that all of its vertebrse, save the most pos- 

 terior ones, are longer than broad. Cimoliasaurus is, there- 

 fore, a relatively short-necked plesiosaur, though not so short 

 as in the genus JPolycotylus. Nothing is known of the pec- 

 toral girdle or of the skull_ of Cimoliasaurus, and more deci- 

 sive characters ma} 7 be, — I believe will be, forthcoming when 

 these parts of the type species are known. Unfortunately, 

 here as among the mosasaurids, it may be a long time before 

 the subject is cleared up finally. I am, however, firmly of the 

 opinion that the two genera are distinct, and therefore object 

 to the indiscriminate use of Cimoliasaurus in the way that it 

 has been used, both in this country and in Europe. 



While we may assume the distinction between Cimolia- 

 saurus and Elasmosaurus, we can by no means do so for the 

 genus Erimosaurus, described long ago by Leidy from a 

 dorsal vertebra from the Cretaceous, probably Benton, of 

 Clark county, Arkansas. 1 believe that this, genus will com- 

 prise species now located under Elasmosaurus, and possibly, 

 if not probably, the type species of that genus. For the 

 present, however, we do know pretty nearly what Elasmo- 

 saurus is, and I shall therefore use this name as the designa- 

 tion of at least ten species of the genus known to me, so far as 

 they can be distinguished by true generic characters. I should 

 perhaps except one of the species described below, Elasmo- 

 saurus (f ) marshii, because I am of the opinion that it will 

 eventual!} 7 be necessary to locate it elsewhere. Indeed I 

 should do this now, did I not feel doubtful of its relationships 

 to some other, as yet poorly distinguished genera. 



Elasmosaurus. 



The type species of Elasmosaurus was founded by the late 

 Professor Cope, and based upon a specimen ascribed to the 

 Niobrara Cretaceous of western Kansas, in the vicinity of old 

 Fort Wallace. From the locality given for the type specimen 

 I long ago concluded that its horizon was really basal Fort 

 Pierre Cretaceous, and not Niobrara ; and an examination of 

 the type specimen, now in the Academy of Natural Sciences 

 of Philadelphia, to which much of the original matrix yet 

 adheres, confirms this determination. Though the type speci- 

 men included a large part of the skeleton, yet through some 



