S. W. Williston — North American Plesiosaurs. 221 



Art XVI. — North American Plesiosaurs : Elasmosaurus, 

 Cimoliasaitrus, and Polycotylus ; by S. W. Williston. 

 (With Plates I-IY.) 



During the past two years I have had the opportunity of 

 studying nearly all the specimens of plesiosaurs preserved in 

 the American museums, a study undertaken in the preparation 

 of a monographic revision of the American forms, and, it is 

 hoped, of the genera of the world also. The accumulation, 

 however, of material recently has made the completion of the 

 task a more arduous one than was at first suspected. I have 

 therefore determined to publish from time to time the more 

 important results obtained, with the hope eventually of gather- 

 ing the whole together in a final monographic revision. Fur- 

 thermore, I am convinced that specimens of this order of 

 reptiles are not as rare in America as has been believed, and 

 hope to continue field search until the more important charac- 

 ters of the group have been established. I have already said, 

 and I repeat, that the taxonomy of the plesiosaurs is very per- 

 plexing ; I doubt if that of any other order of reptiles is 

 more so, chiefly because of the fragmentary nature of much 

 of the known material. 



I desire in this place to express my sincerest thanks to those 

 gentlemen who have generously aided me by the communica- 

 tion of material under their charge, and in particular to Dr. 

 Witmer Stone of the Philadelphia Academy of" Sciences, 

 Professor Henry F. Osborn of the American Museum of New 

 York City, President Slocum and Professor Cragin of Colo- 

 rado College, and to mj old friend, Mr. William H. Reed, 

 curator of paleontology of the University of Wyoming. I 

 am especially grateful for the generosity with which Professor 

 Charles Schuchert, the curator of the Geological Department 

 of the Yale Museum, has placed freely at my disposal the rich 

 collections of that museum, collections which 1 had, for the 

 most part, assisted in making a good many years ago ; and to 

 which were added many useful notes made by the late Pro- 

 fessors Marsh and Baur, and by myself while an assistant in 

 that museum twenty years or more ago. Professor Marsh had 

 begun, before his death, the critical study of the plesiosaur 

 material of the Yale Museum, and had had much of it pre- 

 pared and some illustrations made. All of this has been 

 placed at my disposal. Professor Marsh had not definitely 

 determined any of his species, and had only tentatively located 

 some of them in genera, aside from the Jurassic species de- 

 scribed by him as Pantosaurus striatus. Most of the observa- 



Am. Jour. Sci.— Fourth Series, Vol. XXI, No. 123.— March, 1906. 

 16 



