50 HAY. [Vol. II. 



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 discs, and the union of the 

 remaining 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 aborted, 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 

 the top of the pleurocentrum ; when the latter was pushed for- 

 ward beneath the neural arch to take the place of the aborted 

 epicentrum, this newly developed bone was carried along and 

 was thus brought between the pleurocentrum and the base of 

 the neural arch. 



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

 Xiphactinus supporting the true neural arch is simply the 

 epicentrum itself, aborted, indeed, in Amia, nevertheless per- 

 sisting in Xiphactinus, but crowded upward out of its original 

 seat on the notochord. 



Either 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 has been derived from either the bases 

 of the upper arches or the pleurocentra. The deep gashes in 

 the vertebral centra of Xiphactinus,. where the arches have 



