No. I.] THE GENUS OF FOSSIL FISHES. 31 



prootic. The sutures between the adjoining edges of each two 

 of these bones meet in the apex of the fossa. The axis of the 

 fossa is directed inward and upward. Without doubt, the fossa 

 in the side wall of the skull of Xiphactinus was essentially the 

 same as that in Tarpon. 



In Tarpon there is, as has already been mentioned, an exten- 

 sive fossa on each side of the skull, excavated principally in the 

 basioccipital. This is so deep that only a thin wall of bone 

 separates that on the right side from that on the left. Each 

 fossa is continued forward on the outer surface of the prootic, 

 becoming narrower and shallower. It is across this fossa that 

 the broad process of the opisthotic is thrown as a bridge. A 

 somewhat similar fossa existed in Xiphactinus, but on account 

 of the compression suffered by the skulls its features cannot be 

 definitely determined. 



The prootic of Xiphactinus, like that of Tarpon (Fig. i, pro), 

 provides a portion of the articular surface for the head of the 

 hyomandibular. In Tarpon there are on the external surface 

 of the prootic some four or five foramina. In Xiphactinus I 

 have been able to detect only one of these, that for probably a 

 branch of the facial nerve. It lies just below the anterior end 

 of the articulation of the hyomandibular, and corresponds to 

 that marked 7' in the figure of Tarpon. In Tarpon this fora- 

 men opens into a canal which runs backward in the prootic and 

 emerges at the hinder border of the mouth of the fossa, above 

 described as being walled in by the prootic, opisthotic, and 

 exoccipital. This canal is then continued backward on the 

 outer surface of the exoccipital beneath the opisthotic. It — 

 or, at least, its hinder portion — serves to conduct the glosso- 

 pharyngeal nerve. An opening has been found in Xiphactinus 

 in the mouth of the fossa, and doubtless the canal was similarly 

 prolonged both forward and backward. 



Crook's statement that the parasphenoid is triangular in 

 section, with the base of the triangle directed upward, is true 

 only when the skull is held in an inverted position. The error 

 is doubtless due to a slip of the peni It is also erroneous to 

 say that the finger-shaped processes outstanding from each side 

 of the parasphenoid arise at the union of the parasphenoid and 



