182 



S. \V. WILLISTON. 



might be supposed to be the prosquamosal, but in the excellent 

 skulls of four genera of these animals which I have examined 

 there is positively no such separate element present, nor was 

 there any recognizable prosquamosal present in the skulls de- 

 scribed by Andrews. Nor has such an element ever been recog- 

 nized in the Nothosauria. I am aware that Koken l has suspected 

 the presence of such an element in one specimen of a nothosaur, 

 but his evidence was very doubtful and has not been confirmed. 

 I am confident that the true squamosal in these groups, as in 

 the cotylosaurs and turtles, articulates directly with the post- 



FiG. 17. Ichthyosaurus, after Owen. 



orbital and jugal, without the intervention of any other element, 

 and that the prosquamosal is wanting, not fused with adjoining 

 bones ; nor is there any certain evidence of the presence of the 

 quadratojugal in any of these reptiles. In certain plesiosaurs I 

 have found and figured what I believed to be a distinct ossifica- 

 tion --a small, perhaps rudimentary one that I took to be the 

 quadratojugal. But I have never been able to trace it out, and 

 no other observer has ever distinguished this element in the 

 Sauropterygia. In the Anomodontia no such element has been 

 certainly found, and, since we have the best of reason for believ- 

 ing that the anomodonts and theriodonts are closely related to 

 the ancestors of both the nothosaurs and the plesiosaurs, we can 

 hardly expect to find the bone distinct in these latter forms. 



1 Zeitschr. Deutsch. Geolog. Gesellsch., XLV. , 1893 ; p. 362. 



