FOSSIL RAYS. 221 



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 resem- 

 bles that of a moveable mast, raising and lowering back- 

 wards the sail of a barge. 



The common Dog-Fish, or Spine Shark, (Spinax Acan- 

 thias, Cuv.,) and the Centrina Vulgaris, ha.ve a horny ele- 

 vator 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 farther use as oflfensive and defensive weapons 

 against voracious fishes, or against larger and stronger in- 

 dividuals 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 cartilagi- 

 nous 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 jux- 

 taposition in the same stratum, that their respective species 

 can be ascertained. 



Fossil 'Rays. 

 The Rays form the fourth family in the order Placoi- 



* Colonel Smith saw a captain of a vessel in Jamaica who received many 

 severe cuts in the body from the spines of a Shark in Montego Bay. (See 

 Griffith's Cuvier.) 



The Spines of Balistes and Silurus have not their base, like that of the 

 spines of Sharks, simply imbedded in the flesh, and attached to strong mns- 

 cles; but articulate with a bone beneath them. The Spine of Balistes also 

 is kept erect by a second spine behind its base, acting like a bolt or wedge, 

 which is simultaneously inserted, or withdrawn, by tiie same muscular ma- 

 lion that raises or depresses the spine. 



19* 



