INTRODUCTION 



There arc few orders of reptiles, so long and so widely known as 

 arc the plesiosaurs, of which our knowledge is more unsatisfactory. 

 It has been within the past decade only that a tolerably complete 

 knowledge of any form has been obtained, thanks largely to the 

 researches of Seeley, Dames and Andrews. Especially is our ignor- 

 ance of the American forms yet great. Very few figures or adequate 

 descriptions have been published of our numerous and diverse types. 

 Not only are the specific characters of the descriptions almost wholly 

 undecipherable, but the generic characters even can be satisfactorily 

 made out in but few. Thirty-two species and fifteen genera have 

 be< d described from the United States, and in not a single one of 

 them has there been even a considerable part of the skeleton made 

 known. The skull is known in but three species, and in only one has 

 there been any description of it. With the exception of a sketch of 

 the incomplete girdles of Elasmosaurus plaiyurus t and of a few limb 

 bones by Leidy, with an outline figure of a Megalneusaurus paddle by 

 Knight, nothing of the extremities has been published. And yet, 

 specimens of plesiosaurs are not at all rare in American deposits and 

 collections. 



Although most of the genera and species of the United States 

 have been founded on such scant material, and even more scanty 

 descriptions, that their identification is almost impossible, except by 

 actual comparison of the type specimens, it is not at all improbable 

 that nearly all the names which have been proposed will eventually 

 be found valid. The group has a wide geological range, from the 

 Jurassic to the uppermost Cretaceous, nearly every epoch being repre- 

 sented by one or more species. 



The writer has for some time given such attention as his duties 

 permitted to the study of the American plesiosaurs, in the hopes 

 eventually of clearing up much of the confusion now existing con- 

 cerning these animals, and the present paper was intended to be pub- 

 lished as a portion of this monographic study. As, however, the 

 publication of so extensive a paper must be deferred for some time, 

 he has thought best to publish that portion now prepared in advance 



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