266 University Geological Survey of Kansas. 



row of teeth, very irregular in size. The larger of these teeth 

 are very deeply set in the jaw and Professor Hay has recently 

 shown 24 that those near the symphysis descend nearly to the 

 lower border. The pulp cavities are very large at the base and 

 the displacement takes place by the young crown rising within 

 the pulp cavity of the functional tooth and the absorption of 

 the old root. 



In the early work upon this genus, a great deal of stress 

 was placed upon the number, size and arrangement of the teeth 

 in the determination of the species, which characters have since 

 been shown to be very inconstant. 25 The reason for the great 

 inconstancy is easily explained when we consider the fact that the 

 old teeth are being constantly shed and new ones taking their 

 places, thus having fully developed and young teeth on the same 

 jaw. The palatines and pterygoid bones are probably toothless ; 

 at least there are no teeth of any size upon them. The palatine 

 is peculiar in having a malleolar portion connecting the max- 

 illa with the skull. It is connected with the quadrate by means 

 of the pterygoid bones, thus forming a strong arch. 



The skull is provided with a prominent supraoccipital crest 

 posteriorly. The orbital cavity is large* and the orbit is sur- 

 rounded by a heavy sclerotic ring, and also a chain of supra- 

 and suborbital bones. The pterotics form prominent posterior 

 lateral angles of the skull and also furnish the principal sup- 

 port for the hyomandibular. The parasphenoid forms a strong 

 lower axis of the skull and has prominent transverse processes 

 just in front of the brain case. 



The opercular bones are large and thin, and probably all 

 present. The pectoral fin is large and its rays powerful, prob- 

 ably forming weapons of offense and defense. The pelvic fins 

 are supported by the pelvic actinosts, which are heavy, compact 

 bones, strongly united in the median line by means of suture. 

 The vertebrae are mostly two-grooved. The known American 

 species are : 



Xiphacttinus audax Cope, Niobrara Cretaceous, western Kansas. 

 Xiphaatinus lestrio Cope, Niobrara Cretaceous, western Kansas. 



24. Zool. Bull., vol. II, p. 36. 



25. Kans. Univ. Quart., vol. VII, pp. 115-119, pi. VII-X. 



