. Fishes. Fishes. The Class Elasmobranchii or Shark-like Fishes 183 The subclass Holocephali, or Chiniwras, 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 Chimseras than the modem forms have. The Selachii.—There have been many efforts to divide the sharks and rays into natural orders. Most writers have con- tented themselves with placing the sharks in one order (Squall or Galei or Pleurotremi) having the gill-openings on the side, and the rays
. Fishes. Fishes. The Class Elasmobranchii or Shark-like Fishes 183 The subclass Holocephali, or Chiniwras, 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 Chimseras than the modem forms have. The Selachii.—There have been many efforts to divide the sharks and rays into natural orders. Most writers have con- tented themselves with placing the sharks in one order (Squall or Galei or Pleurotremi) having the gill-openings on the side, and the rays in another (RajcB, Batoidci, 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 {Hexanchidaz) have this structure articulated with the postorbital part of the skull. In the Prosarthri (Hetero- dontidcB) 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, 'n general, the groups thus characterized are undoubtedly natural ones. The sharks are among the earliest fishes to appear in the rocks, and from primitive sharks all the higher groups of fishes are descended. The earliest known and lowest in. Fig. l%^.—Cladoselache fyleri (Newberry), restored. Middle Devonian of Ohio. (After Dean.) Structure constitute the order Plenropterygii iCladoseladiidw), typified by Cladoselache fyleri from the middle Devonian of Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly
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