90 University of Kansas Geological Survey. 



the group has been in existence. Some minor divergent forms 

 have appeared, such as the singular Phosphorosaurus Dollo, 

 Prognaihosaurus, and Brachysaurus, and, perhaps, Baptosaurus 

 Marsh, which, by the way, is one of the latest American forms, 

 from the Upper Greensand or Marl of New Jersey, and occur- 

 ring, also, if Merriam's determination is correct, in the Niobrara 

 of Kansas. 



The common aquatic ancestor of the three types must be 

 sought for in a much earlier period, certainly in the Lower Cre- 

 taceous. The rudimentary or possibly functional zygosphene 

 among the Platecarpinse, or some members of it, and the com- 

 plete zygosphene in Clidastes, together with the shortened 

 muzzle and more fully ossified paddles, indicate a much closer 

 relationship between the Platecarpina? and Mosasaurina? than 

 between either and the Tylosaurinse. In the last we find, in 

 some forms at least, that the fifth finger is actually longer than 

 the fourth, with as many phalanges, and that the carpus and 

 tarsus are almost wholly unossified. If we assume with Dollo 

 that the zygosphene is a primitive character, (and it must be 

 unless it had an independent origin among the Mosasaurs,) then 

 Clidastes would be the most generalized and Tylosaurus the most 

 specialized of the Mosasaurs. In the paddles and skull, Tylo- 

 saurus is, with hardly a doubt, more specialized than any other 

 genus. However, although Clidastes may retain some of its 

 primitive characters, it certainly shows in many other respects 

 a high degree of specialization. 



I give below a tabular review of the known genera of the 

 Mosasaurinse arranged in systematic secpuence, using the Eu- 

 ropean time epochs for comparison's sake. Of course it is un- 

 derstood that the exact equivalency of these time periods is yet 

 a matter of uncertainty. 



TYLOSA IBIX^. 



Tylosaiirus Marsh. 



Cenomanian of New Zealand (Liodon haumuriensis Hector) . 

 Upper Turonian of Kansas and New Mexico (Niobrara) . 

 ? Senonian of New Jersey ( Greensand ) . 



