No. I.] THE GENUS OF FOSSIL FISHES. 27 



Cope's genus Portheus. No more doubt exists in my mind 

 regarding the generic identity of many of the other spines. 

 Some of these, indeed, were found in a block of soft limestone, 

 and were in close relation to jaws, vertebrae, etc., of P. thati- 

 nias. These spines I have compared with Leidy's type of 

 XipJiactimis audax, and I find no difference that can be regarded 

 as generic. Both Cope, in his Ct'etaceoiis Vertebrates, and 

 Crook, in Palaeontographica, vol. xxxix, p. 119, have described 

 and figured spines of Portheus which differ in no essential 

 respect from Xiphactinus. The genus Ichthyodectes possessed 

 pectoral spines not greatly different in structure from those of 

 Portheus ; but none of them attain the size of those assigned 

 to Xiphactinus and Portheus. Taking all the facts into con- 

 sideration, it seems to me that there can be no reasonable 

 doubt that Xiphactinus is the same as Portheus, and ought to 

 supersede it as a name for this genus of fishes. It is quite 

 probable that X. audax is the same as some one of Professor 

 Cope's species of Portheus ; but it will require a careful study 

 of well-identified spines of all the species, and a comparison of 

 them with Dr. Leidy's type specimen to decide the question. 

 For the present, then, we must recognize six American species 

 of Xiphactinus ; viz., X. aitdax (Leidy), X. violossits (Cope), 

 X. thawnas (Cope), X. Icstrio (Cope), X. 7midgei (Cope), and 

 X. loivii (Stewart). 



In my study of the genus Xiphactinus I have been greatly 

 aided by comparison of its various parts with those of the tar- 

 pon of our southern coast {Tarpon atlanticiis). While the 

 tarpon is in many respects quite unlike Xiphactinus, in others 

 it strikingly resembles the latter. Although the two genera 

 undoubtedly belong to different families, these families are 

 closely related, and both belong to the order of Isospondyli. 

 It was in this order that Professor Cope arranged Portheus and 

 its related genera, but he believed that in them he found also 

 characters which indicated relationship with the Siluroids. 

 Such characters I am unable to perceive. Xiphactinus was an 

 Isospondylid, generalized in some respects, but greatly special- 

 ized in others. This specialization shows itself especially in 

 the teeth and paired fins. 



