OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 41 



A proper consideration of the facts deduced from the study of 

 the development and composition of the vertebrae of Amia suffice to 

 give us notions regarding the manner in which the vertebrae of this 

 tish and its ancestors have been produced quite different from those 

 which have hitherto been held. The two bony rings belonging to each 

 muscle-segment of the tail of Amia, and of some other fossil fishes, in- 

 stead of being in one of the last stages of vertebra-formation, needing 

 only to unite in order to form a definitive vertebra, are really in one 

 of the earlier stages. The transformation of the condition found in 

 the tail of Amia to that in the dorsal region, has apparently been 

 effected by the great increase in size of the hypocentral portion of the 

 epi-hypocentrum, the resulting suppression of the haemacentral por- 

 tion ot the pleuro-haemacentrum, the enlargement of the pleurocen- 

 tral portion .of the pleuro-haemacentrum and the concomitant sup- 

 pression of the epicentral portion of the epi-hypocentrum. In 

 other words, the lower portion of the anterior of the two rings was 

 suppressed by the expansion of the lower portion of the hinder ring, 

 while the uppermost portion of the anterior ring grew at the expense 

 of the uppermost portion of the hinder ring. The dorsal vertebrae 

 of Eurycormus, (Zittel, 60, p. 230, Fig. 242) it seems to me, are in a 

 stage of transition from the one condition to the other. That the 

 epicentrum, the ossification at the bases of the upper arch, was at 

 length suppressed, is evident from its not appearing in either fossil 

 or in larval Amias. In the latter the upper arch is not simply lifted 

 up out of contact with the notochord ; its base is aborted, the ossifi- 

 cations which ought to appear there never developing. 



In case this course of transformation has been pursued, there 

 must have been stages in which the sutures between the two elements 

 were oblique or parallel with the axis of the fish, instead of perpen- 

 dicular. And such, indeed, appears to have been the condition in 

 some of the Lepisostei (Pholidopkoruf). Dr. Zittel says of the genus 

 named : "In der hinteren Caudal-region stehendie Hypo-und Pteu- 

 rocentren einander fast gegeniiber und bilden dadurch eine knb'cherne 

 Scheide, auf welcher die untern und oberen Bogen aufsitzen." 



While the course of modification which has just been described 

 has been pursued by many, perhaps most, of the relatives of Amia, 

 and possibly by other fishes, it appears probable that some, such as 

 Cat urns and Callopterus (Zittel, 60, pp. 221 and 230, Figs. 241, 243) 

 followed, so far as they went, a different course. In Caturus the ver- 

 tebral elements often lack much of covering the notochord. Never- 

 theless the lower intercalated elements, as an ossification at least, is 

 absent, and possibly also the base of the upper arch. Evidently no 



