Williston.] Mosasaurs. 211 



preterminal dilatation of the tail, together with the more elon- 

 gate chevrons and their rigid coossification with the centra, all 

 show a more powerful propelling organ than is the case with 

 either of the other genera. 



The differences in the paddles in the three genera are also 

 conspicuous, those in Platecarpus being the largest, and those of 

 Tylosaurus the smallest. The hind paddles in Tylosaurus are 

 the largest and the least reduced. Hyperphalangy is carried to 

 the greatest exfent in Tylosaurus, where the fifth digit of the 

 front paddle, also, is not at all reduced. On the other hand,, 

 the phalanges are least numerous in Clidastes. The flexibility 

 is greatest in Tylosaurus, where mobility is obtained at the ex- 

 pense of strength. In Clidastes the opposite extreme is seen. 

 In the one, control over the different movements through the 

 water was due chiefly to the tail; in the other, to the limbs. 

 In Clidastes the bones of the limbs are all closely articulated, 

 and the tarsus and carpus are fully ossified. In the other 

 genera, and especially in Tylosaurus, the limb bones had a 

 greater amount of cartilage between them and the joints were 

 correspondingly less well formed and less perfect. In Clidastes 

 the ossification of the bones is more complete ; their texture is 

 finer and more solid, and the bones are less liable to distortion 

 or compression. 



Upon the whole, Platecarpus combined the greatest flexibility 

 with the greatest strength, and was, for its size, the most pow- 

 erful and most pugnacious of the Kansas Mosasaurs. In later 

 geological times its prowess was doubtless contested by the 

 species of Mosasaurus proper. In the Kansas seas, however, 

 Platecarpus icteric us was, I believe, the king of the Mosasaurs, 

 though neither the largest nor the most fleet in its movements. 



In size, the maximum among the Kansas Mosasaurs was 

 reached in Tylosaurus chjspelor, which may have attained a length 

 of thirty-five feet, with a head measuring four feet in length. 

 The smallest and most graceful of all was Clidastes pumilus, 

 which had a length of about six feet. A few species larger than 

 T. dyspelor are known from New Jersey, some possibly attaining 

 16— iv 



