THE SERPENTLIKE SEA SAURIANS. 215 



tinct. It is expected that in the Fort Pierre formation of the Dakota 

 region other species will be found, as it has been but imperfectly 

 explored. Europe has about a dozen species, and New Zealand 

 several more. Probably only about forty species of twice as many 

 alleged to have been discovered in the world will stand the test of 

 critical examination. 



Of plesiosaurs, America has produced about ten and the Old 

 World many more species that will stand. Many species of ichthyo- 

 saurs are recorded from Europe, India, Africa, Australia, New Zea- 

 land, and the arctic regions, and one or two in America, the tooth- 

 less Baptanodon from the Jurassic of Wyoming being the type. 

 All three groups had paddles with webbed digits, but none had claws. 

 AVilliston thinks that the ancestors of the mosasaurs were land lizards. 

 Dollo thinks that the ancestors were the peculiar group of lizards 

 which appeared in the commencement of the Cretaceous known as 

 Dolichosauria. Baur would derive the mosasaurs from even more 

 specialized lizards, and believes that their relationship is very close 

 to the monitors of the present day. 



The ichthyosaurs are thought by Cope to be derived from Ho- 

 mceosaurus (beakless lizards) of the Jurassic; and these from the 

 Palwohaiteria (ancient hatteria), a rhynchocephalian (snout-head) 

 which flourished as early as Permian times; and these from the 

 Labitosaurus, an ancestor below the Carboniferous in the Palaeozoic 

 age; from which also sprang the lizardlike saurians, the dimetrodons 

 (Otocoelius), which gave origin to the turtles (Testudinata). Some 

 members of the group to which the plesiosaurs belong were land 

 animals, and hence the origin of the whole group is clearly from land 

 species. It is not now presumed that the marine saurians had much 

 power of progression on land, but they may have climbed on to the 

 beaches to lay their eggs. It is further presumed by Morris that 

 in later times the eggs of saurians were devoured by other animals,* 

 contributing to the extinction of all saurians. 



Three species of representative genera of Kansas mosasaurs have 

 been restored by Williston from material in the University of 

 Kansas. 



Clidastes velox (Marsh) is a typical mosasaur, the perfected 

 skeleton of which is twelve feet in lenth. Pumilus, of the same 

 genus, is given as six feet in length, which would rank it as perhaps 

 the smallest mosasaurian. The clidastes of Kansas had short, power- 

 ful propelling tails, which would indicate a lesser speed than that of 

 their longer-tailed contemporaries. The clidastes had small hind 

 limbs, shomng further deficiency in speed. The animals were 



* Thought by Cope to have been the multituberculate Prototheria. 



