Apr. 1903. North American Plesiosaurs — Wilms ion. 61 



is usually called the presphenoid. Such perfect and persistent 

 ossification of the parasphenoid, scarcely found elsewhere among the 

 reptilia, unless it be the snakes, is in any case remarkable. Why 

 should it be persistent in separating such persistent foramina, unless 

 the openings were of functional importance? One would expect 

 that the pterygoids would have united along the whole median 

 line, as in the Nothosauria, or that they should have closed up 

 in front of the basisphenoid, as in the Chelonia. Sollas objects 

 to this posterior position of the nostrils, because there is no ossified 

 canal for the air passages, as in the crocodiles; but his specimen, as 

 ours, would call for a canal running in the opposite direction, from 

 the posteriorly situated external nares anteriorly to the internal open- 

 ings, and, in the present species, this canal would have been eight or 

 more inches in length. And why may not the canal have been 

 cartilaginous in either case? 



Mandible. — The mandibles are nearly in place on the under side 

 of the skull. They are a little compressed from above downward. 

 The symphysis is short, the two jaws meeting in a considerable angle. 

 The portion in front of the posterior end of the symphysis has been 

 somewhat injured in the specimen, so that the precise shape and 

 length can not be ascertained. The angular extends posteriorly into 

 a relatively short process; the expansion below the cotylar cavity 

 is rounded. In front of the cavity, the angular extends forward to 

 within six inches of the symphysial angle, terminating in a slender, 

 sharp end. From a little in front of the middle of the ramus it is 

 excluded from the inner surface. In front of the cotylar cavity the 

 greater width of the outer surface is composed of the surangular. 

 Between these two bones, the dentary sends a long, slender process 

 backward to within six inches of the cotylar cavity. 



Length of skull to end of mandible . . . . 1.1 m. 



Length of skull to condyle ........9m. 



Width of skull between outer margins of quadrate . . .28111. 



Vertebr/e. — The atlanto-axial complex is thoroughly united, with- 

 out indications of sutural division. The axial rib appears to be 

 united with the axis only, though it may come in contact with the 

 axial intercentrum. The atlas is convex from side to side, without 

 indication of a median carina. The other cervical centra, as Seen 

 from below, are nearly flat, with a slight convexity in the middle, and 

 a slight concavity on either side before the sutural surface for the rib. 

 This flatness is a natural character and not due to compression. The 

 median convexity increases gradually throughout the cervical series. 



