Page:An Introduction to the Study of Fishes.djvu/95

Rh 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.

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