Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/225

 Rh regulated by the elevation or depression of the spine, during the peculiar rotatory action of the body of Sharks. This action of the spine in raising and depressing the fin resembles that of a movable mast, raising and lowering backwards the sail of a barge.

The common Dog-Fish, or Spine Shark, (Spinax Acanthias, Cuv.,) and the Centrina Vulgaris, have a horny elevator spine on each of their dorsal fins, but without teeth or hooks; similar small toothless horny spines have been found by Mr. Mantell in the chalk of Lewes. These dorsal spines had probably a further use as offensive and defensive weapons against voracious fishes, or against larger and stronger individuals of their own species.

The variety we find of fossil spines, from the Graywacke series to the Chalk inclusive, indicates the number of extinct genera and species of the family of Sharks, that occupied the waters throughout these early periods of time. Not less varied are the forms of palate bones and teeth, in the same formations that contain these spines; but as the cartilaginous skeletons to which they belonged have usually perished, and the teeth and spines are generally dispersed, it is chiefly by the aid of anatomical analogies, or from occasional juxtaposition in the same stratum, that their respective species can be ascertained.

The Rays form the fourth family in the order Placoidians.