SUB-CLASS V 



GANOIDEJ 



67 



bony plates (pleurocentra), which are usually pointed below and frequently fused 

 together dorsally, thus forming 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 do 

 not tapei- but continue of uniform width, and meet together 





Fig. 120. 



Two caudal vertebrae 

 of Pyciwdus platessus, 

 Ag. (after Heckel;. 



.1 



li y< 



Fig. 121. 



Vertebrae of Euthynotus (A), and Caturus Jurcatus, Ag. (B). 

 hyc, Hypocentrum ; n, Neural arch; p, Parapophysis ; pic, 

 centrum ; sp, Cleft neural spine. 



'•, Rib; 

 Pleurn- 



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

 notochord (Fig. 121). In many genera (E urycormus) the anterior abdominal 

 region consists of half vertebrae, while 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 



A 



Fig. l •_"_'. 



A , Caudal vertebrae of Eurycorm us speciosus, Wagn. ]:, Verte- 

 braeof Amia calva, Linn. From anterior part of caudal region. 

 h, Haemal arch; hyc, Hypocentrum; n, Neural arch; pic, 

 Pleurocentrum. 



Fig. 123. 



Aspidorhynch us, sp. Caudal 

 vertebrae, each bearing a 

 neural and haemal arch. 



are 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 beny fishes, is 

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

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

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



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

 tin. The true diphycercal condition (see p. 8) constantly persists in some 

 ( Vossopterygians (Coelacanthidae) and Chondrosteans (Belonorhynchidae). In 

 most Crossopterygians the caudal fin is hetero-diphycercal, 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 heteroceraj and 



