100 University of Kansas Geological Survey. 



by suture. The jugal is incomplete. The transverse bone 

 unites with the maxillary and jugal. The pterygoids are with- 

 out teeth. The basipterygoid processes are longer and the 

 pterygoids, hence, much more widely separated. The basi- 

 occipital processes are much smaller ; the exoccipital elements 

 larger. The quadrate is more slender and has no suprastapedial 

 process. The splenial and presplenial interdigitate and do not 

 unite by a distinct articulation, the presplenial extending much 

 further proximally and articulating with the coronoid. The 

 sides of the parietal bone are not decurved to form the sides of 

 the brain case anteriorly. There is a frontal subrhinencephalic 

 bridge. 



COMPARATIVE ANATOMICAL DESCRIPTIONS. 



The skull, in the Kansas forms of the Mosasaurs, is elongate, 

 wedge-shaped, and flattened. The external nares are elongated 

 slits, with an anterior dilatation, and separated from each other 

 by the slender prolongation of the premaxillary and the co- 

 ossified nasals, and, at the posterior narrowed extremity, by the 

 anterior end of the frontals. Externally they are bounded by 

 the prefrontals and maxillae. The orbits are irregular in out- 

 line, broader from in front back than from above downward. 

 Their plane is outward, with a superior and anterior obliquity. 

 Their free margins are composed of the prefrontals, usually the 

 frontals for a short distance, the postfronto-orbitals, the jugal, 

 and the lachrjonal. In Clidastes, and, in a less degree, in Mosa- 

 saurus, the upper part of the orbital cavity forms part of the 

 superior plane of the skull, covered over, in life, by membrane, 

 and supported by the projecting prefrontals, which here func- 

 tionally replace the supraciliare of Varanus. The frontal bone 

 is nearly plane and is unpaired, though there is an indication 

 in all, but especially in Tylosaurus , of the original division into 

 two bones, anteriorly. The supratemporal fossae, directed up- 

 wards, are large, bounded externally by the postfronto-pro- 

 squamosal arch, posteriorly by the parieto-squamosal arch. A 

 pineal foramen is always present, is usually large, and situated 

 near the anterior end of the unpaired parietals. The jugal arch 



