184 S. W. W1LLISTON. 



In all the plesiosaurs (Fig. 16) the union of the squamosal with 

 the postorbital has become much reduced, they merely touching 

 each other and approaching the theriodont type, in which the post- 

 orbital has become wholly separated from the squamosal (Fig. 12). 

 The assumption that the prosquamosal or quadratojugal has 

 thrust itself up between the squamosal and the postorbital is 

 gratuitous, without evidence to support it. And, if they have 

 not been thrust upward into this intercalary position, it must be 

 assumed, if the bones are really present, that they form the 

 lower part of the bar, parallel with the squamosal, intercepting 

 the jugal from union with the squamosal. That the jugal may 

 unite with the squamosal, even when the prosquamosal exists as 



FIG. 15. Stenometopon, after Boulenger. 



an independent ossification, is a fact, as is shown by the structure 

 in the cotylosaur skull. Why then is it necessary to assume that 

 these bones, or either of them, are present in the anomondonts 

 and sauropterygians in a fused condition ? 



It is of course possible that the quadrate has also become a 

 part of the mammalian arch, as has been urged by Dollo, Al- 

 brecht, Baur and others. The extraordinary development of the 

 squamosal bone in the anomodonts and theriodonts has not only 

 crowded out the prosquamosal and quadratojugal, but has also 

 caused the absorption of the quadrate, enclosed between it and 



ward development than is usually seen in the Cynodontia ; but the difference between 

 the groups is not due to any difference in the nature of the arches, but to a less de- 

 velopment of the quadrate bone in the Theriodontia, which has resulted in a diminu- 

 tion or atrophy of the descending pedicle of the squamosal bone" (Seeley, Phil. 

 Trans., 1894, p. 997.) 



