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POPULAR SCIENCE MONTHLY. 



slender, witli short heads. Tlie vertebrse were firm, closely articu- 

 lated with the best system of interlocking of any of the mosasaurs. 

 The limbs were flexible and strong, with closely articulating bones 

 and fully developed tarsus and carpus. The aggregate of these 

 characters indicates the most snakelike form and method of pro- 

 gression through water of all the mosasaurs. The genus Cli- 

 dastes was founded by Cope in 1869, but may ultimately give 

 way to the genus Mosasaurus of Conybeare. Cope's views of Cli- 

 dastes conclude that the animals were not as large as those of the 

 genus Liodon (Owen), but more elegant and flexible, with an addi- 



Restokation of the Plesiosaur by Prof. S. W. Williston. 



tional pair of articulations at either end of each vertebra — the zygo- 

 sphenes — to prevent dislocation by contortions. A larger and still 

 more elegant species was Clidastes tortor (Cope), with lithe move- 

 ments which enabled it to capture fish by means of its knife-shaped 

 teeth, Avliich were very numerous. Tortor was very slender, with a 

 long and lance-shaped head. It was upward of twenty feet in length, 

 with a head two feet and a half long, the vertebral column elongate 

 and the head narrow and pointed. 



The second-type mosasaur perfected by Williston is Platecarpus 

 coryphceus (Cope). Its special characteristics are a short muzzle, 

 slender vertebrae, and an imperfect interlocking zygosphene. The 

 hind paddles are smaller than those forward, but thought to have 

 been more powerful propelling functions than those j)ossessed by 

 other genera. A type skeleton measures fourteen feet, and may 

 have been a young animal. The teeth were very curved and pointed, 

 and formed effective weapons. The neural spines, not closely con- 

 nected, indicate flexibility. The general characters suggest a power- 



