538 



5. W. WILLISTON 



of the right. The specimen, therefore, has very 

 Httle that is conjectural about it save the terminal 

 phalanges of the paddles. Of the more complete 

 specimen the vertebral column lay with every vertebra 

 articulated and in position from the skull almost to 

 the extreme tip of the tail, the last one preserved 

 measuring about eight millimeters in diameter. Of 

 the many hundreds of specimens of mosasaurs which 

 \ I have collected, I have seen but very few with the 

 // extreme tip of the tail preserved in position; the 

 small nodular terminal centra, feebly attached in 

 life, are almost always dispersed. Of this series of 

 vertebrae' seven are cervical, twenty-three are dorsal, 

 the first non-costiferous vertebra being the twenty- 

 fourth. This is one more than I have found posi- 

 tively in other specimens, and in the specimen 

 mounted, and is precisely the number I found in 

 specimens of Tylosaurus proriger; while Osborn has 

 ^ recorded twenty-two as the number found by him 



f v\a\^ in a specimen of Tylosaurus dyspelor. Of the caudals, 

 ij/ six are pygals, one more than I have found in speci- 

 mens of Platecarpus coryphaeus. Seventy caudal 

 vertebrae were preserved in position, to which perhaps 

 six or eight terminal nodular ones may be added, mak- 

 ing eighty-five or eighty-six in all, about the number 

 estimated by me as characteristic of Platecarpus. 

 The distal caudals have, it is seen, a distinct elevation 

 of the spines, a character I have never seen in 

 other specimens of Platecarpus. Osborn has figured 

 the tail of Tylosaurus dyspelor,^ with a distinct 

 elevation of the distal spines. His figures and state- 

 ments do not need corroboration; the character 

 certainly exists in the figured specimen. On the 

 other hand, v. Huene^ has recently figured specimens 

 which he refers to the same species, which do not 



^ Memoirs American Museum, I, Pt. IV, 178. 



2 Geologische paleont. Abhandlimgen (1910), VIII, 297^ 



