Williston.] Mosasaurs. 123 



rated from the exoccipital. The bone articulates with the pa- 

 rietal, supraoccipital, exoccipital, basioccipital, basisphenoid, 



and squamosal. 



Squamosal. 



Clidastes. The squamosal is a small bone, firmly wedged in 

 between the prosquamosal and paroccipital, as also the petrosal. 

 It articulates broadly on the outer side with the prosquamosal, 

 sending a more or less elongated process along the upper inner 

 side of that bone, which may touch the posterior end of the 

 postfrontal. A long, thinned and curved process is directed 

 upwardly and inwardly to unite with the distal extremity of 

 the parietal process, completing the parieto-squamosal arch. 

 Internally it is broadly and firmly united with the exoccipital 

 of Cope, the paroccipital of Baur, extending inward on the 

 anterior face. In none of the specimens in the museum is the 

 petrosal separated from the exoccipital, so that it cannot be 

 said with certainty that the relations of the parts in this genus 

 are like those of Platecarpus, but such is doubtless the case. 

 Below, it forms the middle, antero-posterior, elongated portion 

 of the quadrate articular surface, which is completed on the 

 outer side by the prosquamosal, and, to a very slight extent, in 

 some cases at least, by the paroccipital. The bone articulates 

 with the paroccipital, petrosal, parietal, prosquamosal, and, in 

 some cases to a limited extent, with the postfrontal. 



In Platecarpus the squamosal closely resembles that of Cli- 

 dastes. The internal process on the paroccipital extends very 

 nearly to the semicircular canals, forming a long, slender, pyra- 

 midal process firmly wedged in between the exoparoccipital and 

 the petrosal, and completely excluded by them from the external 

 surface. The parietal branch is rather longer, and there is no 

 process extending on the prosquamosal. 



In Tylosaurus the internal process is probably like what it is 

 in Platecarpus, but this cannot be determined. There is some- 

 times a slender process on the prosquamosal, as in Clidastes, 

 though never reaching the postfrontal. 



