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, {199}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. [Illustration: 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.)]