INTRODBUGEION: 
There are few orders of reptiles, so long and so widely known as 
are 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 
been 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 Flasmosaurus platvurus, 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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