The Chondropterygians exhibit a most extraordinary diversity in the development of their vertebral column; almost every degree of ossification, from a notochord without a trace of annular structure to a series of completely ossified vertebræ being found in this order. Sharks, in which the notochord is persistent, are the Holocephali (if they be reckoned to this order, and the genera Notidanus and Echinorhinus). Among the first, Chimæra monstrosa begins to show traces of segmentation; but they are limited to the outer sheath of the notochord, in which slender subossified rings appear. In Notidanus membranous septa, with a central vacuity, cross the substance of the gelatinous notochord. In the other Sharks the segmentation is complete, each vertebra having a deep conical excavation in front and behind, with a central canal through which the notochord is continued; but the degree in which the primitive cartilage is replaced by concentric or radiating lamellæ of bone varies greatly in the various genera, and according to the age of the individuals. In the Rays all the vertebræ are completely ossified, and the anterior ones confluent into one continuous mass.
In the majority of Chondropterygians the extremity of the vertebral column shows a decidedly heterocercal condition (Fig. [31]), and only a few, like Squatina and some Rays, possess a diphycercal tail
The advance in the development of the skeleton of the Chondropterygians beyond the primitive condition of the previous sub-classes, manifests itself further by the presence of neural and hæmal elements, which extend to the foremost part of the axial column, but of which the hæmal form a closed arch in the caudal region only, whilst on the trunk they appear merely as a lateral longitudinal ridge.
Fig. 32.—Lateral view.
Fig. 33.—Longitudinal section.
Fig. 34.—Transverse section of Caudal vertebra of Basking Shark (Selache maxima). (After Hasse.) a, Centrum; b, Neurapophysis; c, Intercrural cartilage; d, Hæmapophysis; e, Spinal canal; f, Intervertebral cavity; g, Central canal for persistent portion of notochord; h, Hæmal canals for blood-vessels.
The neural and hæmal apophyses are either merely attached to the axis, as in Chondropterygians with persistent notochord, the Rays and some Sharks; or their basal portions penetrate like wedges into the substance of the centrum, so that, in a transverse section, in consequence of the difference in their texture, they appear in the form of an X.[6] The interspaces between the neurapophyses of the vertebræ are not filled by fibrous membrane, as in other fishes, but by separate cartilages, laminæ or cartilagines intercrurales, to which frequently a series of terminal pieces is superadded, which must be regarded as the first appearance of the interneural spines of the Teleostei and many Ganoids. Similar terminal pieces are sometimes observed on the hæmal arches. Ribs are either absent or but imperfectly represented (Carcharias).