292 University Geological Survey of Kansas. 



Coming now to the anterior region of the vertebral column 

 of Amia, we find that each vertebra is formed through the 

 suppression of certain of the elements which, in the tail region, 

 constitute the vertebral rings or disks, and the union of the re- 

 maining elements of each muscular segment into a single mass. 

 The lower intercalated cartilages are suppressed. The upper 

 intercalated cartilages hypertrophy, and their ossifications unite 

 with the bones developed in the bases of the lower arch, thus 

 giving origin to the centrum. The ossification that we might 

 expect to find developing in the base of the cartilaginous neural 

 arch, the epicentrum, is absorbed, while the ossification of the 

 enlarged intercalated cartilage, the pleurocentrum, pushes 

 itself into the place of the epicentrum, and thereafter supports 

 the neural arch. 



Now, we have the choice of two suppositions, neither of which , 

 however, may be the true one. We may hold that a distinct 

 bone was developed in the somewhat elongated and projecting 

 intercalated cartilage, and this, of course, rested on top of the 

 pleurocentrum ; when the latter was pushed forward beneath 

 the neural arch to take the place of the aborted epicentrum, 

 this newly developed bone was carried along, and thus brought 

 between the pleurocentrum and the base of the neural arch. 



Or, we may hold that the bone which I have found in Xiphac- 

 tinus supporting the neural arch is simply the epicentrum itself, 

 aborted, indeed, in Amia, nevertheless persisting in Xiphactinus , 

 but crowded upward out of its original seat on the notocord. 



Neither of the above suppositions presupposes that the upper 

 half of the vertebral centrum takes its origin from the pleuro- 

 centrum. Professor Cope held that the vertebrae of fishes are 

 "intercentra " — that is, have originated in the suppression of 

 all the other elements through the excessive development of the 

 hypocentra. But the very existence, in many genera, of a 

 cartilaginous X in a transverse section of the centrum is proof 

 that its upper portion was derived from either the bases of the 

 upper arches or the pleurocentra. The deep gashes in the ver- 

 tebral centra of Xiphactinus, where the arches have fallen out. 

 furnish evidence that this cartilaginous X was present. 



