34 



HAY. 



[Vol. II. 



they were quite as extensive in Xiphactinus, and composed of 

 very thin bone. Crook has figured them as extending well back 

 from the orbit, and I find them pressed down on the metaptery- 

 goid and hyomandibular of X. thaicmas. 



Dr. Crook states that the ossified sclerotic of Xiphactinus 

 forms a complete ring, meaning, I take it, that it does not con- 

 sist of more than one piece of bone. Having a portion of the 

 sclerotic in my possession which closely resembles those figured 

 by Professor Cope {Cret. Vert., PI. XL, Fig. 3; PI. XLIII, Fig. 

 4), I am inclined to believe that the sclerotic consisted of two 

 separate pieces of bone, and this is the usual condition of the 

 sclerotic of fishes. 



No one has yet, so far as I know, described the nasals, and 

 I have not succeeded in identifying them. In Tarpon each of 

 these bones is a rugose scale which lies partly on the outer 

 border of the ethmoid. It might easily become detached dur- 

 ing maceration, and this accident may have happened to this 

 bone in the skulls of Xiphactinus that I have examined. 



The maxillaries and the premaxillaries are 

 the most characteristic bones of this genus, 

 and especially on the number and the charac- 

 ter of the teeth borne by them have been 

 founded most of the different species. Fre- 

 quently, however, the premaxillary has been 

 separated from the maxillary. I believe that 

 the species may be identified from the con- 

 dyles of the maxillary. At least, these con- 

 dyles are quite different in the two species 

 which I have been able to examine, X. 

 thanmas and X. violossiis. Fig. 2 represents 

 the maxillary of the former species. Fig. 3 

 that of X. vwlossus. From these figures it 

 will be seen that in X. tJiaimias the posterior 

 YxG. z.-xiphacti7ius condyle (p.mx.c.) is notched behind, while 



^ArtMWrtj, maxillary and j \r 



premaxillary, seen from that of P. VloIoSSUS is CXCaVatcd iu frOUt. 



^ '■ It appears, too, that the condyle is more 



extended longitudinally in X. thanmas, more transversely in 

 X. mo/ossjis. Examining the anterior condyle, the one which 



