OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 45. 



other, and their union is a secondary matter. In Rhodeus the coales- 

 cence of the two sets of arches has probably occurred as a result of 

 their being crowded close to each other, the result of the lifting- up of 

 the lower arches to give room for the contents of the abdomen. 



8. FURTHER CONSIDERATION OF THE VERTEBRAL COLUMN OF LIVING 

 AND EXTINCT AMPHIBIA. 



It may now prove profitable to examine a little more closely the 

 condition of the vertebral column of the Amphibia, living and extinct,, 

 in the light furnished by the embryology of Anita. 



In the tail of Archegosaurus, and also in the two posterior presa- 

 cral vertebrae of Chelydosaurus, there occur, according to H.von Meyer 

 (47, p. 104) and Fritsch (Fauna der Gaskohle), basal pieces which 

 Fritsch has called hypocentra pleuralia. These have already been 

 referred to, and the name haemacentra proposed for them. We find 

 then in the Stegocephali the same four vertebral elements that are 

 found in Amia during its larval condition. Furthermore, Fritsch tells 

 iis that in the tail of old specimens of Arctygosaurus there are indica- 

 tions of embolomerous structure of the vertebrae. This might very 

 naturally be brought about by the coossification of the bases of the 

 upper and lower arches to form one centrum, and the union of the 

 pleurocentra with the haemacentra to form the other. The identity 

 of such a condition with that found in the middle tail region of Amia 

 would be complete. But should such double vertebrae not occur in 

 Archegosaurus, they are found in the genera , Cricotus and Diploverte- 

 bron. Prof. Cope tells us that such vertebrae are found throughout 

 the vertebral column of Cricotus. It seems to me that this arrange- 

 ment must have arisen here as it does in Amia, and that the ancestors 

 of the amphibians possessed both upper and lower intercalated car- 

 tilages throughout the length of the vertebral axis. 



On the composition of the vertebrae of the Lepospondyli, with 

 their tubular vertebrae, our studies so far throw but little light. Pos- 

 sibly we may understand them better after an examination of the 

 vertebral structures of the living amphibia. In the Branchisauridae 

 the thin tubular centra consist, as already stated, each of two lateral 

 halves, separated dorsally and ventrally by sutures. We have here 

 evidence to the effect that the vertebral rings are primitively not 

 simple. 



It is a matter of the highest interest to explain, if possible, the 

 morphogeny of the vertebrae of the higher fishes, the amphibians- 

 and the Amniota. Probably in the case of none of these groups shall 

 we be able to do this satisfactorily. In the attempt to elucidate the 



