S. W. Williston — North American Plesiosaurs. 225 



they are distinguishable by more than generic characters, for 

 the family Elasmosaurus is a distinct one, though its charac- 

 ters are not quite those assigned to it by Professor Seeley. I 

 may add that so far, from a prolonged study of the American 

 specimens and descriptions, I am of the opinion that no single 

 species of American plesiosaurs can be placed in any known 

 European genus. 



Unfortunately, the type specimen of Elasmosaurus no 

 longer has the girdles described by Cope. What has become 

 of them is not known. There are some parts of these, espe- 

 cially the clavicular arch, that are necessary for a correct under- 

 standing of the genus. However, from an attentive study of 

 this type specimen and of several other specimens which can 

 be with much probability, if not certainty, referred to the 

 same species, I am enabled to give the following characters 

 for the genus Elasmosaurus. Those characters derived from the 

 type specimen or type species are given in italics ; those 

 derived from other species referred to the genus, in roman : — 



Elasmosaurus. Symphysis of mandible short ; teeth aniso- 

 dont. JYeck with seventy-six true cervical vertebral and three 

 pectorals, the centra increasing in length to the fifty-eighth, 

 and then decreasing to the dorsals ; thence nearly uniform 

 through the thoracic region ; posterior cervicals and dorsals 

 much wider than high, and wider than long ; spines of verte- 

 bras wide and not high • zyg apophyses •weak. Pectoral girdle 

 with large scapulas meeting each other in the middle line. 

 No interclavicular foramen. 



Coracoids broadly separated posteriorly to the inter glenoid 

 thickening, the posterior end not much dilated. Cervical ribs 

 single-headed. Ischia short. Skull short; parietal crest much 

 elevated ; supraoccipital bones parial ; palatines separated by 

 pterygoids. Cervical vertebrae from sixty to seventy-six in 

 number. Scapulae approaching or meeting in middle line. 

 Propodial bones short; two epipodial bones only, not wider 

 than long; digits much elongated. 



Elasmosaurus platyurus Cope. .Fort Pierre Cretaceous of 

 Kansas. 



A detailed description of this specimen, in completion or 

 correction of that given by Cope, will be given later. Certain 

 measurements and remarks may be appropriate here. 



The very broad, depressed, posterior cervical vertebrae of 

 the posterior third, or say the posterior seven feet, prohibited 

 much motion in the living neck, either vertically or horizon- 

 tally. The motility of the neck practically ceased at the fifty- 

 eighth vertebra. Thenceforward the neck was more slender, 

 very slender toward the head. It was to this part that most, if 



