AM 10 ID EI 



329 



bearing separate arches above and below (Fig. 311). At the 

 junction between the two regions may be found intermediate con- 

 ditions, incompletely divided centra (Fig. 306, D). Each pair of 

 centra belongs to one segment and corresponds to one spinal nerve. 

 They therefore represent one normal centrum divided into a pre- 

 and postcentrum (pleuro- and hypocentrum) (Schmidt [388], 

 Schauinsland [384], Hay [203]). The extinct Amioidei show 

 great variety in the structure of the centrum. Eurycormus has 

 pre- and postcentra as in Amia; but in the anterior region each 

 segment is composed of an upper crescentic piece and a ventral 

 rather larger piece ; the series of dorsal crescents are wedged in 



pic. 



Vertebra of Osteorhachis leedsi, A. S. W. ; Oxford Clay, Peterborough. A, oblique side view 

 from behind ; B, front view, ft.c, hypocentrum ; n.a. neural arch ; nt, apei ture for notochord ; 

 pie, pleurocentrum ; r, parapophysis tor rib. 



between the ventral crescents (Fig. 306, A, B, C). In Caturus the 

 whole length of the vertebral column contains complex centra. 

 In Euthynotus the ventral are much larger than the dorsal crescents. 

 From the structure of Eurycormus it would appear that the dorsal 

 wedges represent incomplete precentra formed chiefly by the inter- 

 dorsals, while the ventral and generally larger wedges represent the 

 postcentra formed by the basiventrals. The basidorsals form 

 typical arches, but in these genera appear to contribute little to 

 the centra. Thus, some of the elements which go to make up a 

 centrum in a typical vertebra (p. 100) seem to remain separate 

 throughout life in these fish. A somewhat similar but not identical 

 splitting of the centrum takes place in the Amphibia. 



The skull and visceral arches in Amia are perhaps less specialised 

 than in any other living Teleostome (Figs. 236-240); the chondro- 



