202 FISHES CHAP. 



pair in front or behind the basi-ventrals (Fig. 116). The product 

 of this fusion is a series of bony vertebrae, each consisting of a 

 biconcave arch-centrum, which includes the fused basal portions 

 of a pair of basi-dorsals and a pair of basi-ventrals. The distal 

 portions of the basi-dorsals form the neural arch, while the rib- 

 bearing parapophyses are lateral outgrowths from the basi- 

 ventrals which otherwise have become merged in the centrum. 

 Finally, the centrum is completed by its fusion with a pair of 

 inter-dorsals and a pair of inter-ventrals. Supra-dorsal elements 

 may also be included as minor contributory factors. The supra- 

 basi-dorsals co-ossify with their basi-dorsals and then unite to 

 form the ordinary unpaired neural spine of most bony Fishes, or, 

 as in Amici, they remain distinct from each other, and are obvious 

 as a double spine. In Zepidosteus these elements co-ossify with 

 the neural arches and form the post-zygapophyses. Supra-inter- 

 dorsals have been identified in the embryo as distinct elements, 

 but their eventual fate is not always known. In Zepidosteus they 

 persist as distinct cartilages in the adult (Fig. 118, A). "Well- 

 developed bony ribs are usually present. The haemal arches ot 

 the tail are formed by the downgrowth of the parapophyses and 

 their ribs, or by the latter alone, and by their ventral union to 

 form haemal spines ; consequently, each arch always includes a 

 pair of costal elements. With such general features in common 

 there are certain notable variations in some of these Fishes, to 

 which brief reference may be made. 



Little is at present known of the development of the 

 vertebral column in either of the only two existing genera of 

 Crossopterygii, Polypterus^ and Calamichtliys, and hence the 

 precise mode of grouping of their vertebral components to form 

 vertebrae is unknown. The condition of the vertebral column 

 in the fossil forms varies greatly in different families, but in 

 none is it so specialised as in the surviving members of the 

 group. In the Devonian Holoptychidae, and even in genera so 

 comparatively recent as the Upper Cretaceous Coelacanth Macro- 

 poma, the persistence of the iiotochord and the absence of centra 

 indicate a very primitive grade of vertebral evolution. The 

 Devonian and Carboniferous Ehizodontidae (e.g. Z'usthenopteron 

 and Rhizodus), on the contrary, seem to have had well-ossified 

 ring-like vertebrae. 



' See Budgett, Trans. Zool. Soc. xvj. Pt. vii. 1902, p. 315. 



