S. W. Williston — North American Plesiosaurs. 231 



saurian character, but it differs from the propodial of any 

 other elasmosaurian known to me, in having an additional 

 facet for a supernumerary epipodial at its distal end. There 

 is, also, an additional mesopodial bone, which is wanting in 

 other species of Elasmosaurus. These characters are, I 

 believe, of generic value, but until the structure of the cora- 

 coid is known I leave the species provisionally in this genus. 

 The femur has a length of 370 mm , and a width of 218 mm . 



Thirty-three vertebrae are preserved with the type speci- 

 men, but as already stated they have suffered much from dis- 

 tortion, and their exact measurements can not be given. They 

 are all clearly from the posterior part of the neck and the 

 dorsal region. Some of the posterior dorsals are missing, 

 though one of the sacrals is preserved. Their characters, so 

 far as they are shown, are clearly elasmosaurian. The ribs 

 are of course single-headed ; the spines are broad and not very 

 high ; the posterior dorsals are more flattened at their extremi- 

 ties, and their articular rims are sharp, with slight crenula- 

 tions. The lengths of the centra are given in millimeters, as 

 approximately as the crushed condition will permit, as fol- 

 lows : — 



Cervicals: 135, 135, 135, 135, 135, 135, 133, 133, 130, 127, 

 125, 125, 125, 120. 



Dorsals: 115, 115, 115, 112, 110, 110, 100, 95, 90, 90, 90, 

 90, 90, 90, 90, 90, 90, 85, 80 (sacral). 



If this species had similar proportions to those of E. platy- 

 urus, its length in life was about fifty feet. 



Elasmosaurus ischiadicus Williston. Niobrara Cretaceous of 

 Kansas. 



Polycotylus ischiadicus Williston, Field Col. Mus. Pub., Geol. Ser., vol. 

 ii, p. 72, pis. x, xxvi, 1903. 



This species, based upon the ischia, ilia, caudal and supposed 

 cervical vertebrae, was originally referred provisionally to the 

 genus Polycotylus by myself, though gravely doubting its 

 correct location there. An excellent specimen in the Yale 

 Museum (N/o. 1130), comprising the front paddle nearly com- 

 plete, a number of vertebrae, and the nearly complete pectoral 

 girdle, seems to be of the same species. I was at. one time 

 inclined to the belief that the specimen represented an unde- 

 scribed species, notwithstanding the resemblance of the ischia, 

 chiefly because of the differences in the structure of the ilia, 

 and because of the characters of the vertebrae which 1 had 

 identified as cervical. That all the centra preserved in the 

 Kansas specimen are pygals seems hardly possible. If any of 

 them are cervicals, the species are undoubtedly distinct. This 

 question, however, I can not decide until I have had an oppor- 



