30 HAY. [Vol. II. 



command ; although, on account of the distortion to which the 

 skulls have been subjected, the determination is not as satisfac- 

 tory as is desirable. Its lower process appears to have been 

 much slenderer than in Tarpon. In Tarpon the lower process 

 of the post-temporal is attached by a strong ligament to the 

 posterior extremity of the opisthotic ; and, if I am correct in 

 my determination of both these bones in Xiphactinus, they 

 were brought into close connection. 



The pterotics (squamosals of most authors) were propor- 

 tionally more extensive bones in Xiphactinus than in Tarpon, 

 and formed a more prominent process at the outer and hinder 

 portion of the skull. Each included, I am satisfied, the area 

 marked by Crook as belonging to the parietal. The pterotics 

 furnished the larger part of the articular surface for the head 

 of the hyomandibular. This surface was essentially as it is in 

 Tarpon (Fig. i, ln)i). 



As regards the prootics. Professor Cope's description {Cret. 

 Vert., p. 185) is not far out of the way, though brief. Dr. 

 Crook is less fortunate when he states that the prootics are 

 small. His error arose, if we may judge from his figure of 

 IcJithyodectes polymicrodiis, from his having carved the opisthotic 

 out of the territory of the prootic. The prootics are really the 

 largest of the otic bones. Professor Cope says that with the 

 pterotic and opisthotic this bone bounded a large foramen. 

 This so-called foramen is not really such, but a deep excavation, 

 or fossa, in the side of the skull. In Tarpon this fossa is an 

 inch deep, and about as much in diameter; and it was quite as 

 large in Xiphactinus. In the latter genus the anterior wall 

 appears not to have been completely ossified, so that, in the 

 skeleton, the fossa probably opened widely into the large cavity 

 which lay above the brain, and which will be described further 

 on. Since the cavity just referred to was in life probably filled 

 with the primitive cartilage, the apparent opening from the 

 fossa into it was merely an unossified part of the prootic. 



In Tarpon the mouth of this fossa is somewhat triangular. 

 Its floor is furnished by the exoccipital and the prootic, its 

 posterior wall by the exoccipital and the pterotic, its roof by 

 the pterotic, and the anterior wall by the pterotic and the 



