No. I.] 



THE GENUS OF FOSSIL FISHES. 



35 



a mxc 



articulates with the ethmoid, we find that in X. tJiamnas it is 



large and elongated, and approaches the posterior condyle 



within a distance equal to half its own length. In X. niolossus 



the condyle is much smaller, regularly oval, 



and far removed from the posterior condyle. 



It is to be expected that the other species of 



the genus will exhibit likewise their distinctive 



characters. I am inclined to believe that these 



condyles underwent some individual variation, 



and they have in many cases suffered distortion 



during fossilization, and this must be taken 



into account. The left maxilla belonsrinof to 



the same individual of X. molossiis from which 



Fig. 3 was drawn possessed an additional con- 

 dylar surface, nearly round and small, just in 

 front of the posterior condyle. It is to be in- 

 ferred that the ethmoids of the various species, 

 and especially the surface of the palatine with 

 which the posterior maxillary condyle artic- 

 ulates, will exhibit characters corresponding 

 to those shown by the latter. 



I call attention to the fact that it is as yet difficult to distin- 

 guish the various species by means of characters furnished by 

 the lower jaws. In the case of X. molossits there are discrep- 

 ancies between Professor Cope's description of the number and 

 character of the teeth and one of his figures. The lower jaw 

 of the type specimen is figured on PI. XXXIX of the Cretaceous 

 Vej'tebmtes and again on PI. XL, Fig. i. The statement is 

 made in the text (p. 195) that there are in all 20 teeth ; but 

 in the figure last referred to there are 2^ teeth represented, 

 and these do not all agree in size either with the statements 

 of the text or with the other figure. The explanation of this 

 discrepancy, evidently, is that the figure on PI. XL has, so far 

 as many of the teeth are concerned, been erroneously restored. 

 X. tJia7imas is said {op. cit., p. 197) to have rather more numer- 

 ous teeth than X. molossiis, and in the specimen described 

 there are said to be 23. I possess two dentaries which I 

 regard as belonging to X. thaumas. In these there are 24 



Fig. 3. — X. jnolossHs, 

 maxillary and pre- 

 maxillary, seen from 

 above, x 5. 



