108 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



temporal bar originally enclosed a lateral temporal opening, which was closed 

 through gradual anteroposterior compression of the bar, owing to the enlarge- 

 ment of the orbit. In this form the bar shows fully as much area on the side 

 of the skull as in many reptilian forms possessing a lateral temporal fenestra. 

 Though the temporal bar has probably been somewhat reduced by enlargement 

 of the orbit, it seems very doubtful whether it ever really enclosed a lower tem- 

 poral fenestra. 



The squaniftsal bone is a tri radiate element, as in other ichthyosaurs, one 

 branch extending forward and outward along the outer border of the superior 

 temporal opening, one forward and inward along the outer side of the poste- 

 rior arm of the parietal, and one reaching down to meet the pterygoid on the 

 anterior plate of the quadrate. In a lateral view of the skull the squamosal 

 represents only a small part of the exposed area of the temporal bar, as the 

 lateral anterior arms are largely covered by long posterior processes of the 

 postfrontal, which reach almost to the upper posterior angles of the skull. The 

 large supratemporal is apparently triangular, the acute anterior angle extending 

 forward between the postorbital and postfrontal. The superior margin in con- 

 tact with the squamosal and postfrontal forms a nearly straight line. The in- 

 ferior border is in broad contact with the quadratojugal as in Ichthyosaurus. 

 In Baptanodon this contact is very narrow 7 , and the temporal bar is exceedingly 

 narrow at this point. The quadratojugal extends downward almost to the 

 articular surface of the quadrate and is covered above by the supratemporal. 

 Inferiorly it reaches forward over the inferior temporal notch to meet the 

 jugal. The lower side is considerably thickened a short distance above the 

 border. 



The occipital region of this species (pi. 5, fig. 1) is less satisfactorily known 

 than the other parts of the skull, as the difficulties of preparation have been 

 relatively great. In specimen 9950 most of the elements of the occipital region 

 seem to be in nearly their original position, but the exact form of some of the 

 bones cannot be definitely determined. In specimen 9913 the skull has been 

 broken through obliquely in the occipital region, and the structure of some of 

 the elements is more clearly shown. 



The basioccipital has a peculiar and characteristic form in that its artic- 

 ular face for contact with the atlas is deeply concave instead of convex as" in 

 Ichthyosaurus. In articulation the broad concave face of the basioccipital re- 

 ceived the strongly convex anterior face of the atlas. In specimen 9950 the 

 deep concavity on the posterior face of the basioccipital extends out to the 

 lateral margins. Inferiorly the posterior end of the basioccipital is rounded, 

 and is truncate downward and forward as if for the accommodation of a hypo- 

 centrum. On specimen 9913 the concave face extends- almost to the lateral 

 margins, and the inferior side of the concavity is bounded by a prominent 



