236 KANSAS UNIVERSITY SCIENCE BULLETIN, 



its ventral edge and this is always broken. The complete form 

 cannot therefore be determined from the material now on 

 hand. The most complete specimen in this museum is shown 

 in figure 2. This is the same specimen figured by Stewart ('00, 

 plate 44, figure 5) . It is therefore apparent that the opercular 

 apparatus of Xiphactinus is of the typical teleost form and pre- 

 sents no peculiarities of importance. 



MANDIBLE. 



Cope, Crook, Hay and Stewart have described the mandible 

 of Xiphactinus, and the main points of its structure have been 

 well presented. The only questions that have arisen concern 

 the articulars and the angular. From the material at my com- 

 mand I am able to state with considerable certainty the form 

 and extent of these elements. An angular is present and is 

 united suturally with the dermaticular principally, but is 

 slightly in contact also with the autarticular and the dentary. 

 Not infrequently, however, it has become separated entirely 

 from its contacts, and in that event its limits are clearly indi- 

 cated by the strongly roughened sutural surfaces of the remain- 

 ing bones. In figure 3 is shown a mandible with the angular 

 lacking. It is therefore seen that this element is a small, 

 irregularly triangular structure, and not large, as Cope de- 

 scribed it. Hay figured it ('98, p. 36) as a part of the dermar- 

 ticular. Cope ('75, p. 195) as a portion of the articular (=^ der- 

 marticular). 



With the removal of the angular the nature and extent of the 

 dermarticular is at once apparent, and it may be traced for- 

 ward into the long process attached to the inner side of the 

 dentary. This portion of the bone is regarded as belonging to 

 the angular by Cope ('75, p. 195) and to the autarticular by 

 Hay ('98, p. 37). There can be no question regarding the 

 identification of this element in the specimen before me. The 

 dermarticular is therefore a large bone, extending fully two- 

 thirds the length of the mandible, and from the ventral edge to 

 the coronoid process. The "sword-shaped process" is merely 

 a thickening of the bone on its ventral edge, and above this it 

 suddenly thins out very much. Into the groove thus formed 

 there rests the autarticular, a short, wedge-shaped bone of ir- 

 regular outline, whose broad base affords the main articulating 

 surface of the mandible with the quadrate. In front of the 

 autarticular, and resting in the same groove on the inner sur- 

 face of the dermarticular, is a small, triangular-shaped bone 



