40 HAY. [Vol. II. 



on account of the relatively greater depth of the head in 

 Xiphactinus, the width of its hyomandibular has about the 

 same ratio to its length that we find in Tarpon. 



In Tarpon the process for articulation of the operculum pro- 

 jects from the hinder border of the bone more than in Xiphac- 

 tinus. In the latter genus the surface for the operculum of 

 X. inolossHS scarcely passes beyond the border of the bone ; 

 but in X. tJiaiimas the surface is at the extremity of a consid- 

 erable process. 



As a result, perhaps, of its large size, the hyomandibular of 

 Xiphactinus, as well as that of Tarpon, is provided with promi- 

 nent ridges and depressions, and with foramina leading into its 

 interior. Many of these are repeated in the two genera with 

 much faithfulness. In both genera there is found running 

 down near the middle of the outer surface of the bone a high 

 crest, like the spine of the human scapula. This crest has its 

 origin, we may say, in two low rounded ridges, one beginning 

 at the anterior end of the hyomandibular head, the other at its 

 posterior end, the two ridges converging and meeting opposite 

 the articular surface for the opercular. Here the resultant 

 crest becomes much more elevated, thin, and sharp, and con- 

 tinues to the lower end of the bone. The plane of the crest is 

 directly outward and slightly backward. Both in front and 

 behind the crest is a deep fossa, the posterior one the best 

 defined. The anterior border of the preopercular occupies a 

 part of the posterior fossa. This fossa, in Tarpon, ends above 

 in a deep depression immediately in front of this process for 

 articulation of the opercular ; but from the upper border of 

 this depression one or more large canals enter the bone, and, 

 passing upward, emerge by several mouths in another depres- 

 sion on the inside of the bone just below the head of the 

 hyomandibular. It is quite probable that one or more 

 branches of the facial nerve pass downward through these 

 canals. 



In Xiphactinus the posterior fossa ends above, just as de- 

 scribed, and broad channels are seen passing upward from it 

 in the bone ; while on the outside, just below the anterior end 

 of this hyomandibular articulating surface, there is a depression 



