OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 39 



the more recent investigations are rather opposed to the doctrine that 

 the elastica does become calcined. It is quite evident that primitively 

 it does not. 



6. FURTHER CONSIDERATION OF THE VERTEBRAL COLUMN OF FOSSIL 



FISHES. 



It becomes now proper to consider the composition of the verte- 

 brae of the larval Amia in connection with the fossil members of the 

 group and those of the stegocephalous Amphibians. 



In doing so we cannot fail, it seems to me, to recognize in the 

 upper intercalated cartilages and the ossifications arising from them, 

 the pleurocentra, the bones which cover in the upper surface of the 

 notochord in both the ancient Amioid fishes and the Stegocephali. In 

 a form like Caturus (Zittel, 60, p. 138, Fig. 146) we find the pleuro- 

 centrum occupying a position between the bases of the upper arches 

 just as do the intercalated cartilages in the tail of Amia. In Gallop- 

 terus the upper arches repose on the pleurocentra, out of contact with 

 the hypocentrum, as they do in the dorsal region of Amia. If this con- 

 clusion is correct, and the homologising of the intercalated cartilages 

 with those of Acipenser and Poly o don is justified, we reach the impor- 

 tant result that the pleurocentrum, which plays such an important 

 part in some fishes, amphibians, and all Amniota, has developed from 

 the upper intercalated cartilages of the lower fishes. 



In the tail of our young Amia these pleurocentra furnish only 

 the upper half of the bony material which enters into those vertebral 

 centra which have no arches. The explanation which has hitherto 

 been given of these centra is, that the lateral extremities of the pleu- 

 rocentra have continued to grow downward around the notochord 

 until they have met and coalesced on its under side. From Amia we 

 learn that this is not, at least always, the case. Two other elements 

 in each muscle-segment contribute to the formation of this vertebral 

 ring; these are the two lower intercalated cartilages and their ossifi- 

 cations. In fishes no distinct bones which correspond to those lower 

 elements appear yet to have been discovered ; but in the stegocepha- 

 lous genus Arehegosaurus, Hermann von Meyer (47, p. 104) has de- 

 scribed as being found in each vertebra a distinct ossification which 

 occupies the space between the bases of adjacent lower arches, and 

 on the lower side of the notochord. This ossification Fritsch (Fauna 

 der Gaskohle) calls the hypocentrum pleurale. In the tail of Archc- 

 gosaurus, according to Hermann von Meyer, this bone is divided into 

 two lateral halves. In this condition it corresponds in every respect 

 to the two ossifications in each segment, which result, in an early 



