Williston.] Mosasaurs. 169 



there were about five phalanges in the first and fifth toes, and 

 eight or nine in the others. The phalanges are more slender in 

 this form than in the others, and somewhat less constricted. 

 The fifth digit is evidently divaricated, and undergoing reduc- 

 tion, in both of which characters it differs from that of the 

 front foot. A restoration of the paddle, as I think it should be, 

 will be found in the restoration of Tylosaurus. 



SYSTEMATIC DESCRIPTIONS. 



For the sake of completeness and clearness I give herewith 

 the group and generic descriptions of the known European and 

 American Mosasaurs, rather briefly, however, save for the 

 Kansas forms. 



The determination of the species described by early authors 

 is in large part clearly impossible in the absence of the type 

 specimens. Species after species have been named, based largely 

 or entirely upon mechanical distortions and mutilations. The 

 conditions of petrifaction in the Niobrara seas were such that 

 the bones of these animals are rarely preserved in their natural 

 shape. Vertebrae are almost invariably flattened, and, if pre- 

 served in a vertical position, the bones are depressed; if in a 

 horizontal position, compressed. This distortion is not readily 

 perceived in many specimens, as cracks and breaks are usually 

 wanting, the bones having yielded, as though of a plastic ma- 

 terial. For this reason, very little or no dependence can be 

 placed upon the shape of the centra. In fact, the centra in all 

 the forms appear to have greatly resembled each other in shape 

 during life — a rounded or slightly depressed shape in the cervical 

 region, a more depressed or transversely oval in the dorsal, a 

 subpentagonal or pyriform shape in the pygal, and a vertical 

 oval in the caudal region. Just what is the norm of each genus 

 or species it is difficult to say, and will always remain more 

 or less of a problem. For this reason I have entirely ignored 

 this character in the specific determinations. 



In the restoration of the skull of the different genera a vast 

 deal of study has been necessary to correct the malformations 

 due to fossilization, the skulls fossilized in different positions 



