No. I.] THE GENUS OF FOSSIL FISHES. 



41 



like that found in Tarpon. There can be no doubt, therefore, 

 that the upper end of this hyomandibular is hollowed out simi- 

 larly in the two genera. In a specimen of X. molossiis before 

 me, two bridges of bone are thrown across the upper end of the 

 posterior fossa on the mesial surface of the hyomandibular. On 

 the same surface of the hyomandibular there is a well-marked 

 median crest, in front of which is a broad shallow fossa. It is 

 in the upper end of this fossa that the depression is found that 

 has just been described, and which in Xiphactinus is represented 

 in Fig. 7, dp. This depression, it is to be noted, faces the 

 deep fossa which has already been described as occurring in 

 the side wall of the skull. Its significance can only be deter- 

 mined by an examination of a fresh Tarpon. Both depressions 

 probably furnish insertions for muscles. 



The opercular of this genus is not well known. Cope states 

 that it is thin and broad. Crook figures a portion of the bone, 

 but this reaches downward only about to the middle of the 

 preoperculum. I have a fragment of a bone 50 mm. by 100 

 mm. which appears to be the opercular of X. thaiimas, and this, 

 too, has every appearance of ending about halfway down the 

 preoperculum. This piece of bone has an articular surface 

 resembling that of Tarpon for connection with the preopercular, 

 and, like Tarpon, there are just below this surface, and on the 

 inner side of this bone, one or two large openings into the 

 interior of the bone. This mention of the opercular may attract 

 attention to it. It appears rather improbable that it is really 

 so short as above described. 



In each of the three specimens of Xiphactinus before me 

 there is present, attached to the posterior outer angle of the 

 skull, a bone which seems to occupy the position of the post- 

 temporal. If such it is, it was very different from that of 

 Tarpon. It is not much over an inch in length, and less than 

 two inches broad, but very thick. In a specimen of Tarpon 

 the bone is rather thin and much longer. In Xiphactinus, on 

 account of the crushed condition of the skulls, the relations of 

 the bone are hard to make out, but it seems to be connected 

 with the opisthotic and the epiotic. In many fishes the post- 

 temporal bone is very short and stout. 



