Stewart.] Cretaceous Fishes, 291 



possibly, that the basal pieces are the proper arches, while the 

 pieces which are borne on them are the spinous processes. I 

 hold that there are two objections to this view. The first is, that 

 what are sometimes called spinous processes are always un- 

 paired pieces. The second is, that when the lateral halves of 

 the arches remain distinct from each other and are prolonged 

 into spines, as they are in various fishes — Amia and Salmo, for 

 instance — the spinous portion is never, so far as we know, de- 

 veloped in the embryo as pieces separate from the base of the 

 arcula. This is true in the case of Amia, which I have investi- 

 gated. We must, therefore, seek some other explanation. The 

 key to the understanding of the problem is, it seems to me, to 

 be found in the vertebral column of that primitive fish, Amia. 

 We may call this iisli to our assistance, since the Isospondyli are 

 believed to have ancestors not far removed from Amia. 



In the middle region of the tail of Amia there are, for each 

 muscular segment, two vertebral rings, the one bearing the 

 arches, upper and lower, the other archless. If a transverse 

 section be taken through the arch-bearing ring, there will be 

 found an X of cartilage, the upper arms of which are continu- 

 ous with the cartilage of the neural arch. In like manner, the 

 lower arms will seem to be continuous with the cartilage of the 

 haemal arch. If a section is made similarly through the arch- 

 less disk, a similar X of cartilage will be found, but the arms 

 project beyond the outer surface of the disk but a short distance. 

 These archless disks are developed in Amia from ossifications 

 :ng in the intercalated cartilages, upper and lower, and the 

 arms of the X are the unossified portions of these cartilages. 

 There appears to be no reason why these intercalated cartilages 

 should not sometimes take on a hyper trophied growth. In the 

 sharks they often become considerably larger than the neural 

 arches themselv 



In case these intercalated cartilages should become thus en- 

 larged and arch -like, each might develop a bony investment 

 that would simulate the bony neural half-arch, and thus would 

 on tho top its proper epicentrum. 



"10. For fiu'niv'< illustrating the architecture of the vertebral column of Amia, see tho May 



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