136 'MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



become directly converted into cartilage and, in the higher 

 forms, into bone; but the little we know of development, to- 

 gether with the conditions occurring in the ganoids, and espe- 

 cially in certain fossil amphibia (stegocephali), make it probable 

 that a vertebral body or centrum is more complicated than it 

 was once thought to be. 



The most complicated condition known is found in the fossil 

 Archegosaurus. Here there occurs on the dorsal surface of the 



FIG. 141. Diagram of rhachi- 

 tomous vertebrae, based on Arche- 

 gosaurus. /ia, haemal process ; hc t 

 hypocentrum arcale ; hp, hypo- 

 centrum pleurale ; np^ neural pro- 

 cess ; ns, neural spine ; /, pleuro- 

 centrum ; z, zygapophysis. 



FIG. 142. Trunk vertebra of 

 extinct stegocephalous Eurycormus 

 speciostiS) showing rhachitomous 

 condition, after Zittell. h, hypo- 

 centrum ; n, neural arch ; p, pleuro- 

 centrum; r, radialia. 



notochord on either side between two successive neural processes 

 (Fig. 1 41 ) a skeletal plate, the pleurocentrum. On the ventral 

 surface, opposite the base of the neural process, is an arched 

 band, the hypocentrum * (or hypocentrum arcale), which extends 

 across the notochord from one side to the other. Behind this 

 and opposite the pleurocentra are a pair of skeletal plates, the 

 hypocentra pleuralia. More usually (Fig. 142) the hypocentra 

 pleuralia are absent. These forms belong to the rhachitomous 

 type of vertebrae. 



In the embolomerous type (Fig. 143) a vertebral body is com- 

 posed of two rings, one of which is directly opposite the base of 



1 The terms centrum and intercentrum often used for these parts lead to unnecessary 

 confusion ; the intercentrum is in most cases the hypocentrum arcale. 



