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. [Illustration: Fig. 32. Fig. 33. Fig. 34. 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.]