Apr. 1903. North American Plesiosaurs — Williston. 39 



length. The posterior ribs are short, less flattened, with a less 

 expanded head and with a pointed distal extremity. From the 

 position of the diapophyses anteriorly, it is quite evident that the ribs 

 were directed much more outwardly than downwardly. In the 

 restored skeleton, the transition from the long to the short ribs has 

 been made gradual, and two or three on each side have been 

 modeled here. (See PI. XI.) 



About fifteen ventral ribs were preserved; others have undoubtedly 

 been lost. Only one is symmetrical; it is about thirty centimeters in 

 length and is thickest near its middle, tapering to a point on each 

 side, and has a moderate curvature. The others preserved measure 

 from twelve to sixteen centimeters, and have a thickness near their 

 middle of about ten millimeters. They are gently curved, irregularly 

 prismoidal in section, and taper to a point at each extremity. Because 

 of their number and the scattered positions in which they were found, 

 it has not been possible to arrange them in the mounted skeleton with 

 any degree of precision. In the known European specimens they 

 form a double series of three, with a larger symmetrical median one. 

 Abdominal ribs are known only in th.e crocodilia, rhyncocephalia 

 pterosaurs, ichthyosaurs, dinosaurs and plesiosaurs. 



The caudal vertebrae were found at some little distance from the 

 remainder of the skeleton, and for the most part had been collected 

 separately by Mr. Sternberg. The processes, as elsewhere in the 

 column, had been separated, and were, many of them, found variously 

 intermixed with the other bones. Both the centra and the proo 

 however, were in excellent condition, as were those of the neck, in 

 marked contrast to the dorsal vertebras. The last twelve taper so 

 uniformly that it is quite certain they all belonged together, and that 

 no intervening ones were missing. This series has a diameter of 

 forty millimeters at the beginning and only twelve at the extremity, 

 with lengths respectively of twenty-five and twelve millimeters. The 

 greatest decrease in size occurs in the last five or six of the series. 

 These distal five or six have the shortening much more pronounced 

 on the dorsal than on the ventral side, indicating a well-marked 

 upward curvature of the extremity of the tail. Because of the rapid 

 change in the characters of these vertebrae there has been no difficulty 

 of associating the processes with their respective vertebrae. The 

 spines in the early ones of the series slope at a moderate angle back- 

 ward, the obliquity being greatest in the twelfth from the end. In 

 the last six, the spines are much shorter and stand nearly vertically, 

 or even with an anterior slope. The diameter of the last centrum 

 preserved indicates the presence of two, or perhaps three smaller 



