210 University of Kansas Geological Survey. 



had a few phalanges more or less, but certainly not enough to 

 perceptibly modify the general form. The only parts conjectural 

 in the restoration are the number of the thoracic ribs and the 

 precise number of the caudal vertebrae with chevrons. There 

 are no characters present in this or any other genus by which 

 the number of thoracic ribs may be determined save by their 

 actual preservation in situ. That they were materially different 

 from what is figured, is impossible, since many of the short ribs 

 are preserved in some of the specimens. Isolated caudal verte- 

 brae and partly connected series are present in other specimens, 

 from which it is evident that the tail agrees in its general char- 

 acters well with that of Tylosaurus. They may have been a few 

 more or less of the small vertebra?, but certainly not enough to 

 perceptibly modify the length in the restoration. 



Tylosaurus is based almost wholly upon three specimens in 

 the museum, one Avith the posterior part of the head and the 

 absolutely complete series of vertebra?, connected from head to 

 tip of tail, collected by E. P. West ; the second, with the skull 

 and cervical vertebra? equally complete, obtained from Mr. H. 

 T. Martin ; the third, with the paddles nearly complete, together 

 with many ribs and vertebra?. This last specimen is the one 

 collected by Professor Snow, in which the skin is preserved as 

 figured in plate lxx. 



Each of the three animals thus restored has its own peculiar 

 characters, representing three distinct types of the group, which 

 I have already defined. The skull of Platecarpus is the broad- 

 est ; that of Tylosaurus the slenderest. The jaw teeth are most 

 numerous in Clidastes; the fewest and most powerful in Platecar- 

 pus. The pterygoid teeth, on the other hand, are the strongest 

 in Tylosaurus ; smallest and least effective in Platecarpus. Cor- 

 related with these dental peculiarities .are the large size of the 

 paddles in Platecarpus, and the small size in Tylosaurus. The 

 skull of Platecarpus has a more rounded contour posteriorly, and 

 the striking size of the quadrate is conspicuous. 



In Clidastes the slenderness of the body, the shortened and 

 small thorax, the much greater length of the lumbo-dorsal re- 

 gion, and the proportionally shorter tail are all noticeable. The 



