Stewart.] Cretaa - F 265 



IchthyodecU*! and ir may later be found that the two genera are 

 identical. However, as Cladocyclu* is so imperfectly known, it 

 would hardly be safe to say that the two genera are synonymous. 



Xiphactinus. 



hactinm Leidv. Proc. Acad. Nat. Sci. Phil. 1S70, p. 12; Rep. U. S. 

 Geol. Surv. Terr. 1S73. p. 290, pi. xvn. 

 Portheu* Cope. Proc. Am. Phil. Soc. 1S71, p. 17"): Cret. Vert. West, pp. 

 190-193. 



The genus A was first described by Leidy H from the 



remains of a pectoral fin-spine from the Cretaceous deposits of 

 Kansas. Three years later the spine was more completely de- 

 scribed and figured by the same author in his Contributions to 

 the Extinct Fauna of the Western Territories, i p. 290, pi. XVI T. | 

 Professor Cope recognized the relationship of this spine to his 

 family S " . id a paper published in Hay den's Second 



Annual Report of the Geological Survey of the Territories. 1871, 

 I 418,) but issigned it to the genus Sait . and always 



claimed that Xi was distinct from P - and Iehtl 



although he never stated in what way they differed. 

 It has since been shown, without doubt, that Poriheus is a syno- 

 nym of Xi the latter name having the priority by 

 about a year. 



This genus is represented in the museum by two specimens 

 which are more than twelve feet in length each, and these are 

 only medium-sized individuals : so there can be but little doubt 

 that the species attained a length of twenty or more feet in the 

 largest spec: - -ides their large size, the genus is well char- 



acterized by the tooth-bearing elements, which are all covered with 

 a powerful dentition. The premaxillse are firmly united with 

 the maxillae and usually have one or more large, fang-like teeth 

 projecting downward from the alveolar border. The maxilla- 

 are large and provided with a single row of teeth which vary 

 _ \tly in size. The upper anterior portion of the bone is pro- 

 vided with two condyles which servo to bind the jaw to the 

 ill. Tho dentaries are deep and also provided with a single 



. Proc. Acad. Nat. Sci. Phil. 1- 

 18 



