No. I.] 



THE GENUS OF FOSSIL FISHES. 



51 



fallen out, furnish evidences that this cartilaginous X was 

 present. 



The anterior neural arches of Xiphactinus, probably all of 

 those belonging to the abdominal region, are very different 

 from those of the tail. One of these abdominal neural half- 

 arches, as seen from without, is presented in Fig. 1 5 ; another, 

 seen from the mesial side, is given in Fig. 16. These neural 

 arches are coossified neither with the vertebral centra nor with 

 their fellow bones. The base is hemispherical and planted in 

 a broad excavation in the upper surface of the centrum. The 

 two excavations of each centrum are close together, and it 

 seems probable that the juxtaposed borders of the right and 



art. surf. 



art. surf. 



Fig. 15. Fig. i6. 



Fig. 15. — X. thauitias. Neural arch near head, seen from without, x \. 

 Fig. 16. — X. thanniiis. Neural arch near head, seen from within, x \. 



left elements of each arch are in contact both below and above 

 the neural canal. Behind the base of the neural half-arch is 

 a broad smooth surface (Fig. 16, ai't. surf.) looking mesially, 

 and in life coming into contact with a similar surface on the 

 anterior end of the next vertebra behind and looking outward 

 (Fig. 15, art. surf.). These surfaces remind us strongly of the 

 zygapophyses of the higher animals. 



At the base of each of these half-arches we find a strong rod- 

 like process directed outward and backward. These processes 

 are the epineurals (Fig. 15, e.p.n). They were confluent with 

 the bases of the arches just as they are in Alosa and Tarpon. 

 It is entirely probable that Xiphactinus and its allies were as 

 bony fishes as our just-mentioned modern genera. 



The excavations for the insertion of the neural arches are 

 broadest toward the region of the head. Farther backward 



