Most of the Elasmobranchs are large, coarse-fleshed, active animals feeding on fishes, hunting down their prey through superior strength and activity. But to this there are many exceptions, and the highly specialized modern shark of the type of the mackerel-shark or man-eater is by no means a fair type of the whole great class, some of the earliest types being diminutive, feeble, and toothless.
Subclasses of Elasmobranchs.—With the very earliest recognizable remains it is clear that the Elasmobranchs are already divided into two great divisions, the sharks and the Chimæras. These groups we may call subclasses, the Selachii and the Holocephali, or Chismopnea.
The Selachii, or sharks and rays, have the skull hyostylic, that is, with the quadrate bone grown fast to the palate which forms the upper jaw, the hyomandibular, acting as suspensorium to the lower jaw, being articulated directly to it.
The palato-quadrate apparatus, the front of which forms the upper jaw in the shark, is not fused to the cranium, although it is sometimes articulated with it. There are as many external gill-slits as there are gill-arches (5, 6, or 7), and the gills are adnate to the flesh of their own arches, without free tips. The cerebral hemispheres are grown together. The teeth are separated and usually strongly specialized, being primitively modified from the prickles or other defences of the skin. There is no frontal holder or bony hook on the forehead of the male.
The subclass Holocephali, or Chimæras, differ from the sharks in all this series of characters, and its separation as a distinct group goes back to the Devonian or even farther, the earliest known sharks having little more in common with Chimæras than the modern forms have.
The Selachii.—There have been many efforts to divide the sharks and rays into natural orders. Most writers have contented themselves with placing the sharks in one order (Squali or Galei or Pleurotremi) having the gill-openings on the side, and the rays in another (Rajæ, Batoidei, Hypotrema) having the gill-openings underneath. Of far more importance than this superficial character of adaptation are the distinctions drawn from the skeleton. Dr. Gill has used the attachment of the palato-quadrate apparatus as the basis of a classification. The Opistharthri (Hexanchidæ) have this structure articulated with the postorbital part of the skull. In the Prosarthri (Heterodontidæ) it is articulated with the preorbital part of the skull, while in the other sharks (Anarthri) it is not articulated at all. But these characters do not appear to be always important. Chlamydoselachus, for example, differs in this regard from Heptranchias, which in other respects it closely resembles. Yet, in general, the groups thus characterized are undoubtedly natural ones.
Fig. 298.—Fin-spine of Onchus tenuistriatus Agassiz. (After Zittel.)
Hasse's Classification of Elasmobranchs.—In 1882, Professor Carl Hasse proposed to subdivide the sharks on the basis of the structure of the individual vertebræ. In the lowest group, a hypothetical order of Polyospondyli, possibly represented by the fossil spines called Onchus, an undivided notochord, perhaps swollen at regular intervals, is assumed to have represented the vertebral column. In the Diplospondyli (Hexanchidæ) the imperfectly segmented vertebræ are joined in pairs, each pair having two neural arches. In the Asterospondyli or ordinary sharks each vertebra has its calcareous lamella radiating star-like from the central axis. In the Cyclospondyli (Squalidæ, etc.) the calcareous part forms a single ring about the axis, and in the Tectospondyli (Squatina, rays, etc.) it forms several rings. These groups again are natural and correspond fairly with those based on other characters. At the same time there is no far-reaching difference between Cyclospondyli and Tectospondyli, and the last-named section includes both sharks and rays.