38 HAY. [Vol. II. 



the dermarticulare. I know of no recent fish which possesses 

 such an arrangement. The tarpon has a very different articu- 

 lation in this region, since it resembles closely the articulation 

 between two vertebrae of a bird. The advantages of such 

 an articulation as that of Xiphactinus are obvious, since this 

 species doubtless preyed on large fishes, and possibly on some 

 of the large aquatic reptiles of its era. Fig. 6 represents 

 another specimen of the jaw of Xiphactinus (No. 1646, U. S. 

 N. M.). At q.c. is seen the condyle of the quadrate ; ep.h. 

 is the lower end of the epihyal, and c.h. the upper end of the 



Fig. 6. — X. vmlossus, showing quadrate, autarticulare, and liyoid bones, x \. 



ceratohyal. These bones are closely appressed to the inner 

 surface of the quadrate and of the lower jaw. 



The entopterygoid, or mesopterygoid (Fig. 7, vi.pg.), has a 

 smooth, slightly convex surface sloping inward and upward to 

 form a partial floor for the orbit. Unless its width has been 

 excessively altered by pressure, it was much narrower than the 

 corresponding surface of Tarpon. In the latter the entoptery- 

 goid meets the upper anterior angle of the quadrate, these two 

 bones thus excluding the ectopterygoid from contact with the 

 metapterygoid. In Xiphactinus, on the contrary, these two 

 last-mentioned bones have a considerable union (Fig. 7). 



The bones of the palato-quadrate arch have been described 

 as being devoid of teeth. I have, however, found a consider- 



