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 Amia, they remain distinct from each other, and are obvious 
as a double spine. In Lepidosteus 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 Lepidosteus 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 Calamichthys, 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 J/acro- 
poma, the persistence of the uotochord and the absence of centra 
indicate a very primitive grade of vertebral evolution. The 
Devonian and Carboniferous Rhizodontidae (e.g. Husthenopteron 
and Rhizodus), on the contrary, seem to have had well-ossified 
ring-like vertebrae. 
1 See Budgett, Trans. Zool. Soc. xvi. Pt. vii. 1902, p. 315. 
