TTilliston.] % Mosasaurs. 141 



Caudal Vertebra?. 



Immediately following the thirty-fifth costiferous vertebra in 

 Clidastes, the tubercular rib-process gives place to an elongated 

 non-costiferous process. There are seven such vertebrae in this 

 species — with elongated flattened process and without chevrons. 

 A distinctive name for them is needed, and in a previous paper 53 

 the name pygial was proposed. These pygial vertebrae, or pygals, 

 are seven in number in this species. The under surface is some- 

 what flattened, and, as in the preceding vertebra?, is gently con- 

 cave antero-posteriorly. The transverse processes are elongate, 

 flattened, with a thin rounded extremity, and are directed gently 

 downward. In the anterior vertebrae the processes spring from 

 the anterior part. As the centra become shorter they arise from 

 near the middle. In the last of the series there are minute in- 

 dications of chevrons. The centra, which had already begun 

 to decrease in length in the posterior part of the lumbo-dorsal 

 series, diminish rapidly, the last being only three-fourths the 

 length of the first. The transverse processes are of nearly the 

 same length throughout, their expanse being fully four times 

 the length of the first centrum. 



The centra of those vertebra? which bear chevrons do not dif- 

 fer much in shape. They become less constricted, and, back of 

 the middle of the series, are smoothly cylindrical in shape. 

 The transverse processes decrease gradually in length and size, 

 and ascend somewhat on the centrum, disappearing entirely on 

 the twenty-fifth or twenty-sixth. The spinous processes are now 

 much narrower than in the precaudal series, though of nearly 

 the same length, increasing gradually in this respect for the first 

 twenty of the series, and are markedly oblique, with the pos- 

 terior border stout, and the anterior border alate. With the 

 twenty-sixth, where the transverse processes cease, the spines 

 begin to increase rapidly in length, and have become more ver- 

 tical in position, with both borders thinner. In the thirty-fifth 

 or thirty-sixth they attain their greatest length, and are here 

 directed slightly forward. Thence to the end of the tail the 

 spines decrease gradually and they become more and more oblique 



55 Kansas Univ. Quart., I, 22. 



