290 BONY SPINES OF SHARKS. 
smooth horny spines connected with the dorsal 
fin. In the Cestracion Philippi alone, (PI. 1, 
Fig. 18), we find a bony spine armed on its 
concave side with tooth-hke hooks, or prickles, 
similar to those that occur in fossil Ichthyo- 
dorulites : these hooks act as points of suspen- 
sion and attachment, whereby the dorsal fin is 
connected with this bony spine, and its move- 
ments 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 moveable 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 vora- 
cious fishes, or against larger and stronger in- 
dividuals of their own species.* 
The variety we find of fossil spines, from the 
Greywacke series to the Chalk inclusive, indi- 
* 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 
