No. I.] THE GENUS OF FOSSIL FISHES. 33 



found in the lower anterior angle of the great lateral fossa; the 

 other is seen just above the foramen 7'. These foramina are 

 probably closed with membrane in life. They are not found in 

 Xiphactinus. 



In Xiphactinus the alisphenoids and the orbitosphenoicTs 

 appear to have had the same extent and relations, at least as 

 seen from below, and I have no doubt that there was in the 

 skull the same large amount of primitive cartilage that we find 

 in Tarpon to-day. 



The frontals of Xiphactinus were much broader than they 

 are in Tarpon. In a tarpon whose skull had to one of 

 Xiphactinus the ratio in length of 9.5 to 10.5, the width of the 

 frontals bore the ratio of i to 2. Since the breadth of the nasal 

 region of Xiphactinus was little less, we may appreciate Pro- 

 fessor Cope's characterization of their expression as being 

 bulldog-like. 



To a broad flat surface of the very stout prefrontal of 

 Xiphactinus was applied the superior articulating surface of the 

 malleolar body of the palatine. In Tarpon the palatine is simi- 

 larly connected with the prefrontal, except that the ethmoid 

 bone sends outward a process which takes part in the articula- 

 tion. Professor Cope states that in the alewife the articulation 

 of the palatine is wholly with the ethmoid. 



The lower surface of the ethmoid furnishes an articular 

 surface for the anterior condyle of the maxillary. Since this 

 condyle in X. thaumas is much larger than that of X. violossus, 

 we ought to find a corresponding difference in the ethmoids of 

 the two species. 



There can be no doubt that the orbit of Xiphactinus was 

 surrounded by a ring of orbital bones, just as it is in Tarpon. 

 In a skull of A', violossus before me (No. 1646, U. S. N. M.), 

 the superorbitals are wanting, but the border of the frontals 

 shows distinctly that a row of thick bones has been articulated 

 with it. In Tarpon there are three of these superorbitals. 

 Crook has figured a preorbital in Xiphactinus. 



In Tarpon the posterior suborbitals are very large, extending 

 backward over the cheek as far as the preopercle. In nearly 

 their whole extent they are membranous. It is certain that 



