OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 21 



of the two centra in the middle tail region. On another page of his 

 paper, when Schmidt comes to consider the fossil forms with rhachi- 

 tomous vertebrae, such as Eurycormus and Euthynotus, he explains 

 these structures on the hypothesis that, starting with the two rings in 

 each myomere throughout the body, the hypocentrum has grown at 

 the expense of the pleurocentrum. The latter becomes reduced in 

 size and is confined to the upper side of the notochord. Eurycormus 

 furnishes one stage ; Euthynotus a more advanced one. At length 

 the pleurocentrum becomes a mere rudiment ; and in Amia, the last 

 of the series, Schmidt finds the pleurocentrum represented by only 

 the small masses of cartilage, "the rudimentary upper arches,," alleged 

 to lie in front of the bases of the developed upper arches. Of course, 

 one or the other, or both, of these explanations must be wrong ; and 

 neither of them explains the arrangement found in Caturus, as is 

 admitted by Schmidt. On the other hand, the theory that the rings 

 have resulted from the growth of the pleuro-and hypocentral plates, 

 seems capable of explaining all the known facts. 



Having considered the mode of development of the vertebrae of 

 the Amioidei, it may be profitable to inquire whether or not any 

 other groups of fishes have possessed similar vertebras. The so- 

 called Ganoids first claim our attention. 



Our modern species of Lepisosteus show no indications in their 

 adult condition of the presence of pleuro-and hypocentra. But Aspi- 

 dorhynchus, a member of the Rhynchodontidae, had dorsal vertebrae 

 composed evidently of two portions. These formed rings, which 

 were divided along the sides by sutures. This being true of the dorsal 

 region, we can hardly floubt that the caudal vertebrae also originated 

 from once distinct pleuro-and hypocentra. The tubular vertebrae of 

 Belonorhynchus, too, must have had a similar origin. 



In the family Macrosemiidae, Cope ^Saurodontidae, Zittel) some 

 genera have simple tubular vertebrae. Others, as Eugnathus; have 

 the vertebrae composed each of two distinct pieces. The species of 

 Pholidophorus sometimes possess undivided vertebrae, but usually the 

 centra are made up of pleurocentra and hypocentra (Zittel, 60, p. 

 315). In the tail these two pieces stand nearly opposite each other 

 and form a bony sheath on which rest the upper and the lower 

 arches. It seems quite improbable that the closely related genera 

 fropterus and Histionotus should have developed vertebrae in an en- 

 tirely different way. 



Little is known concerning the state of the vertebral column of 

 the Sphaerodontidae and the Stylodontidae. At any rate, the ossifica- 

 tions were feeble. Tctragonolepis is said to have had ring- like vertebrae. 



