Fig. 113.—Schematic transverse section through the middle of a Cyclospondylic (A), a Tectospondylic (B), and an Asterospondylic vertebra (C). d, Middle portion of the calcified double cone; d′, additional concentric calcified layers; d″, double cone with radiating calcified layers; ex.m, external elastic membrane; h.a, haemal arch; n.a, neural arch; n.c, notochordal cavity. (From Zittel, after Hasse.)
The mode in which the lime is deposited is marked by certain peculiarities which are characteristic of particular families[[179]] (Fig. 113). In some genera, as in the extinct Palaeospinax and the living Acanthias and Scymnus, the calcified portion of each centrum takes the form of a cylinder constricted across the middle, like two cones joined apex to apex (cyclospondylic). This condition is probably the most primitive, but it may be modified in other genera by the further addition of calcic salts in two different ways. Thus, the deposit may take place by the simple addition of concentric layers to the original constricted cylinder (tectospondylic), as in the Skates and Rays; or it may take the form of a series of longitudinal plates radiating outwards from the cylinder, and giving rise to a star-like pattern in cross-section (asterospondylic), as in Scyllium and Lamna. In most living Elasmobranchs (e.g. Scyllium), but not in such genera as Notidanus, Heterodontus, and Squatina, the bases of the dorsal and ventral arches grow round the centra and meet, or even fuse, so that the latter become surrounded by rings of cartilage which, after a fashion, suggest incipient arch-centra (Fig. 112, A). The caudal portion of the vertebral column is often described as "diplospondylic," that is, there are two centra, two pairs of basi-dorsals, two pairs of inter-dorsals, and two pairs of basi-ventrals, or in other words, two vertebrae to each neuromere[[180]] (Fig. 111, B).
The Holocephali have a vertebral column essentially similar to that of other Elasmobranchs, but of a more primitive type (Fig. 114). The notochord is persistent and there are no centra; but ring-like calcifications, four or five to each neuromere, occur in the chordal sheath in Chimaera, although not in Callorhynchus. Ribs are absent. In the whip-like terminal portion of the tail the arcualia and the notochord become replaced by a slender continuous filament of cartilage.
Fig. 114.—A, transverse section of the vertebral column of Chimaera monstrosa; B, lateral view, c.r, Calcified ring; h.r, basi-ventral; int, inter-dorsal; n.a, neural arch (basi-dorsal); nch, notochord; nch.sh, chordal sheath; n.sp, neural spine (supra-dorsal). (From Parker and Haswell, after Hasse.)
In the more obvious features of vertebral structure the Dipnoi[[181]] have much in common with the Elasmobranchs, especially with certain of the acentrous Palaeozoic representatives of that group. The notochord is persistent, centra are wanting, and the different vertebral components continue to retain their primitive distinctness. On the other hand, the basi-dorsals are much better developed than the inter-dorsals, which are either vestigial or absent. The basi-dorsals unite in pairs over the spinal cord to form complete neural arches, and each arch supports dorsally the legs of a Λ-shaped, gable-like element or neural spine, which probably represents a pair of fused supra-basidorsals. Ventrally, there are basi-ventral cartilages, fused in pairs beneath the notochord, and supporting well-developed, bone-ensheathed ribs. Inter-ventrals appear to be absent. Each neuromere corresponds with a pair of basi-ventrals, of basi-dorsals and of inter-dorsals. The haemal arches and spines are formed partly by the basi-ventrals, but mainly by the ventral union of the successive pairs of ribs. As in the Holocephali, the terminal arcualia of the tail become fused into a straight axial cartilaginous filament, transversely divided into segments, which replaces the notochord. Each segment supports a variable number of dorsal and ventral gable-pieces, or neural and haemal spines. Certain of the vertebral components, such as the ribs, and the neural and haemal spines, are ensheathed by membrane bone.
Fig. 115.—Side view of the precaudal vertebrae of a Sturgeon (Acipenser sturio). a.c, Aortic canal, formed by the median union of ingrowths from the basi-ventrals and inter-ventrals of opposite sides; b.d, basi-dorsals; b.v, basi-ventral; i.d, inter-dorsal; i.v, inter-ventral; n, notochord; n.c, neural canal; n.sp, neural spine; nt.s, cuticular sheath of the notochord; p, parapophysis; r, rib; s.n, aperture for the root of a spinal nerve.
With certain modifications in details the preceding description will also apply to the vertebral column of the Chondrostei (Fig. 115). It will be noted, however, that the inter-dorsals are much better developed than in the Dipnoi, although when compared with the basi-dorsals they take but a small share in forming the walls of the neural canal. Well-developed but somewhat fragmentary inter-ventrals are present. The haemal arches and spines are formed by the downgrowth and ventral union of the basi-ventrals as in the Dog-Fish, and apparently without the aid of costal elements. In Polyodon the ribs are vestigial,[[182]] but in Acipenser they are well developed. The neural arches and spines, and their haemal representatives in the tail, and also the ribs, are partially ossified, or ensheathed by bone.