128 



It, is slated by Professor Marsli that these reptiles possessed scales ; that, 

 in Clidastes, a complex pattern was ])roduccd l)y alternate rows of scutes 

 of different shape and size. They are osseous, but thin, and generally united 

 by beveled edges. Such structures would produce a distinct effect in the 

 living animal. I had already inferred the existence of dermal scales, and 

 figured them in a restoration of Mosasaurus, published in the American Nat- 

 uralist,^ but had not suspected osseous scuta. It is a little singular that I 

 have never detected them among the numerous skeletons of these reptiles 

 which I have exhumed in Kansas and elsewhere ; nor have any been sent me 

 by Professor Mudge. 



The proportions varied somewhat in the known genera. Thus, in 

 Clidastes, we see the greatest attenuation of form ; while, according to Marsh, 

 the caudal series is less elongate in Liodon. In Clidastes and Platecarpus, 

 the humerus is very short and wide, broader than the femur. In iodon, 

 the humerus is more slender, and not very diiferent in proportions from tlic 

 femur; hence, the flippers were more distinctly pedunculate. 



• 



Classification and distribution. 



The well-distinguished genera of the order known from North American 

 strata are the following: 



I. Cervical hypapophyses separate, articulating: 



a. A zygosphenal articulation : 



Chevron-bones coossified with centra Clidastes. 



Chevron-bones free Sironecfes. 



aa. No zygosphenal articulation : 



Teeth subcylindric conic; humerus short, platelike; 



chevron-bones free Platecarpus. 



Teeth mostly compressed, cutting; humerus with nar- 

 rowed extremities ; chevron-bones free Liodon. 



Teeth subcylindric faceted ; chevron-bones in part 



coossified . . Mosasaurus. 



II. Cervical hypapophyses continuous and (nitire : 



No zygosphene Baptosaurus. 



The material obtained in Kansas, during the autunm of 1871, by the 



writer, proved conclusively that this order of reptiles attained a predominant 



'18C0, \^. ai. 



