Williston.] Mosasaurs. 171 



prosquamosal, and post-frontal. A complete parieto-squamosal 

 arch present. Paroccipital united with exoccipital ( ?) . Quad- 

 rate large, free, with a large suprastapedial process, articulating 

 proximally with prosquamosal, squamosal, and sometimes with 

 the exoccipital (paroccipital) . Transverse bone small, articu- 

 lating with pterygoid and jugal only. Pterygoids elongate, ap- 

 proximated in front and behind, but not contiguous, united by 

 ligament to the quadrate, and provided with numerous teeth. 

 Palatines small, without teeth. A ball-and-socket joint between 

 splenial and presplenial. Skull elongated and narrow. 



Mosasauridse. 



Moscisauridce Gervais, 1853. 



Clidastidce Cope, Proc. Bost. Soc. Nat. Hist., 1869. 

 Tylosauridce Marsh, Amer. Journ. Sci., xn, July, 1876. 

 JSdestosauridcB Marsh, Amer. Journ. Sci., xxi, p. 55, 1878. 



TYLOSAURIN^E. 



Tylosnurince Williston, Kans. Univ. Quart., vi, A, 180, 1897. 



Hind feet functionally pentedactylate. Trunk short, the tail 

 proportionally long, not dilated distally. Tarsus and carpus 

 almost wholly unossified, the phalanges numerous. Vertebras 

 wholly without zygosphene, or, at the most, very rudimentary. 

 Premaxillary projecting into a long rostrum in front of the 

 teeth. Quadrate with a short suprastapedial process. 



Tylosaurus. 



? Maerosaurus Owen, Journ. Geol. Soc. Lond., 1859, 380. 

 f Lesticodus Leidy, Proc. Amer. Phil. Soc, vn, 10, 1859. 

 ? Nectoportheus Cope, Proc. Amer. Phil. Soc, 1868, 181. 

 Rhinosaurm Marsh, Amer. Journ. Sci., in, 461, June, 1872 (preoc.) 

 Rhamphosaurus Cope, Proc. Acad. Nat. Sci. Phil., 1872, 141 (preoc.) 

 Tylosaurus Marsh, Amer. Journ. Sci., iv, 147, 1872. 



Moderate to large sized species. Rostrum much produced, 

 the nares situated far back. Facial surface of the parietal pro- 

 duced to the posterior part in the middle, the sides nearly par- 

 allel. Postfrontal and prefrontal meeting on the superior 

 orbital border. Prefrontal not expanded on the facial plane 



