SUB-CLAS8 V 



GANOIDEI 



67 



bo!iy plates (pleurocentra), which are usually pointed below and frequently fused 

 together dorsally, thus f orming a second horseshoe-shaped half-ring. According 

 to their size the hypocentra and pleurocentra more or less 

 completely Surround the soft, unsegmented notochord. 

 Sometimes when the upper and lower lateral pieces de 

 not taper but continue of uniform width, and meet together 



i 



i)f 



Fig. 120. 



Two cautlal vertebrae 



J'yciioilus platesstifi, 



Ag. (after Heckel; 



Vertebrae of Kuthyiiotus (A), and Caturu^ furrdivs, Ag. (U). c, llib ; 

 hyc, Hypoceiitruiu ; n, Neural arch ; p, Parapophysis ; [jk; Pleuro- 

 centruiii ; sp, Cleft neural spine. 



dorsally and ventrally, they form two half-rings, which completely enclose the 

 notochord (Fig. 121). In many genera {Earycormus) the anterior abdominal 

 region consists of half vertebrae, w.hile the caudal region consists of ring 

 vertebrae, which are composed of two halves (Fig. 122). By the complete 

 fusion of the two half rings, there arise simple, sheath-like ring vertebrae 

 (Fig. 122). In the Amiidae the vertebral centra of the abdominal region 



hyc . 



h 



Fiti. 122. 

 .-1, Caudal vertebrae of Euri/cormus speciosus, Wagn. B, Verte- 

 brae of Amia calva, Linii. Front anterior part of caudal region. 

 /(, Haenial arch; hi/c, Hypocentrnni ; n, Neural arch; plc, 

 Pleurocentrnrn. 



Aspidorhynchus, sp. Caudal 

 vertebrae, each beariiig a 

 neural and haenial arch. 



ure completely ossified and amphicoelous, while some of those of the caudal 

 region are still divided into two halves, which correspond with the hypo- and 

 pleuro-centra of the Lepidostei (Fig. 122, B). 



The complete ossification of the vertebral column, as in the bony fishes, is 

 observed only among the latest representatives of the Ganoids, namely, the 

 Polypteridae and the Lepidosteidae. Among the latter, moreover, the verte- 

 l)ral centra are only concave behind, convex in front (opisthocoelous). 



The hinder end of the vertebral column is always produced into the caudal 

 tili. The true diphi/cercal condition (see p. 8) constantly persists in some 

 Crossopterygians (Coelacanthidae) and Chondrosteans (Belonorhynchidae). In 

 most Crossopterygians the caudal fin is hetero-dipht/cercal, that is, the vertebral 

 column extends straight, but the rays of the upper lobe of the fin are more 

 delicate and shorter than those of the lower lobe. True Merocercy and 



