NORTH AMERICAN PLESIOSAURS 

 TRINACROMERUM 



S. W. WILLISTON 



The University of Chicago 



In previous papers' I have discussed the known characters of 

 Elasmosaurus, Cimoliasaurus , Brachauchenius and Polycotylus, and 

 Trinacromerum, as derived from the species T. (DoUchorhynchops) 

 oshorni. The brief and somewhat erroneous description of the type 

 of Trinacromerum by its author has hitherto prevented its recognition 

 with much certainty. Its relationships with DoUchorhynchops I 

 recognized at the time that I proposed that genus, and suggested the 

 possible identity, but it was not until I had the opportunity of examin- 

 ing the type specimen of Trinacromerum, which was kindly granted 

 by the president of Colorado College, where the specimen is now 

 preserved, that I became assured of the synonymy, an acknowledg- 

 ment of which was made in the third volume of Chamberlin and 

 SaHsbury's Geology, and, later, in the second of the papers cited below. 

 Furthermore, I am now nearly as well assured of the synonymy of 

 Trinacromerum with Polycotylus, but am yet hesitant to abandon 

 the name Trinacromerum until the skull of the type species of 

 Polycotylus, {P. latipennis Cope) shall be better known and have been 

 more thoroughly studied. The only differences that I can so far 

 discover are the deeper concavity of the vertebral centra, the number 

 of epipodials and the manner of attachment of chevrons — of doubtful 

 value. The name Trinacromerum, therefore, is used provisionally 

 until such time as more positive evidence is forthcoming. 



Very recently, through the courtesy of Professor Sclater, I have 

 had the opportunity of further study of certain important parts of 

 the type specimen of Trinacromerum hentonianum, kindly sent me 

 for that purpose. I am thus enabled to give a number of figures and 

 a more complete description of this important species. 



I Publication No. 7J of the Field Columbian Museum, 1903; American Journal 

 of Science, XXI, 221, 1906; Proc. U. S. Nat. Mus., XXXII, 477, 1907. 



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