Apr. 1903. NoRTH AMERICAN PLESIOSAURS—WILLISTON. 39 
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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 1s 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 Pl? XI.) 
About fifteen ventra/ 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 toa 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 toa 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 the crocodilia, rhyncocephalia 
pterosaurs, ichthyosaurs, dinosaurs and plesiosaurs. 
The cauda/ vertebre 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 processes, 
however, were in excellent condition, as were those of the neck, in 
marked contrast to the dorsal vertebre. 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 vertebre there has been no difficulty 
of associating the processes with their respective vertebre. 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 
