Fig. 116.—Diagram to illustrate the grouping of vertebral elements to form vertebrae, A, in an Elasmobranch, B, in Amia, and C, in Lepidosteus. B.D, Basi-dorsals; B.V, basi-ventrals; I.D, inter-dorsals; I.V, inter-ventrals; in.v.c, inter-vertebral cartilage divided by a concavo-convex cleft; p.c, precentrum; pt.c, postcentrum. The square blocks represent individual vertebrae, and the oblique lines, the attachments of the myocommata.
In the existing Crossopterygii, Holostei, and Teleostei, popularly known as the "bony Fishes," the vertebral column assumes a more familiar character, and at the same time we meet with interesting illustrations of the different methods by which the separate component vertebral elements of the more primitive types of "backbone" are concentrated together in groups, and fused to form that complex physiological product, the complete bony vertebra.[[183]] In most of these Fishes each vertebra is formed by the aggregation and fusion of a pair of basi-dorsals and a pair of basi-ventrals, and includes, in addition, a pair of inter-dorsals, which may either be the pair in front of the basi-dorsals or the pair behind, and also a pair of inter-ventrals, which, similarly, may be the 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 of 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[[184]] 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 Macropoma, the persistence of the notochord and the absence of centra indicate a very primitive grade of vertebral evolution. The Devonian and Carboniferous Rhizodontidae (e.g. Eusthenopteron and Rhizodus), on the contrary, seem to have had well-ossified ring-like vertebrae.
In the caudal region of Amia the basi-dorsals and basi-ventrals, and the inter-dorsals and inter-ventrals, form separate arch-centra which remain distinct; hence each vertebra is double, and there is a regular alternation of arch-bearing "pre-centra" and arch-less "post-centra" (Fig. 117, D). In the trunk-region the pre- and post-centra have fused, and in this region the vertebrae are single.
Fig. 117.—A, precaudal vertebrae of Caturus furcatus; B, similar vertebrae of Eurycormus speciosus; C, caudal vertebrae of the latter species; D, caudal vertebrae of Amia calva. h.a, Haemal arch; h.sp, haemal spine; hy.c, hypo-centrum; n.a, neural arch; n.sp, neural spine; p, parapophysis; p.c, pre-centrum; pl.c, pleuro-centrum; pt.c, post-centrum; r, rib. (After Zittel.)
A very primitive type of vertebral column occurs in some of the Jurassic allies of Amia, in which certain of the vertebral components, confluent in the adult Amia, retain some measure of their primitive distinctness.[[185]] Thus, in the precaudal region of Eurycormus (Fig. 117, B) there is a series of alternating dorsal and ventral half-rings of bone, which between them completely invest the persistent notochord. Each ventral half-ring or "hypocentrum" represents a pair of fused and ossified basi-ventrals, and possibly also a pair of included inter-ventrals, and supports dorsally a pair of basi-dorsals, forming a neural arch, and laterally a pair of ribs. The dorsal semi-rings, or "pleuro-centra," similarly represent fused and ossified pairs of inter-dorsals. In the tail, modifications approximating to what is seen in the caudal region of Amia are to be noticed (C). By the upgrowth of the ventral arch-bearing semi-rings, and their conversion into complete rings encircling the notochord, incipient pre-centra are formed, and by a similar modification of the down-growing, archless, dorsal half-rings, structures comparable to post-centra are produced. In brief, Eurycormus, as well as such other extinct Amioid genera as Caturus (Fig. 117, A), Callopterus, and Euthynotus, retain in the adult a stage of vertebral evolution which is closely paralleled by transitory stages in the embryonic and young forms of Amia.
Fig. 118.—A, two vertebrae from the trunk-region of Lepidosteus; B, anterior face of a vertebra. c.n, Anterior convex face of the centrum; c.n′, posterior concave face; h.a, parapophysis, with its articular facet for a rib; i.c, median cartilage, representing a pair of fused supra-interdorsals; i.s, radial element of the dorsal fin; l.l, superior longitudinal ligament; n.a, neural arch. (From Wiedersheim, after Balfour and W. N. Parker.)