VERTEBRAL C0LU3IN OF FISHES. 69 



from their terminal position ; they are almost always bifurcated, or 

 dichotomously subdivided, as the effect of tlie continued spontaneous 

 fission of their embryonic elements, or of the activity of the vege- 

 tative force of irrelative repetition. This part is accordingly subject 

 to monstrosity by excess, as is manifested by the double and triple 

 tails of Gold-fish in confinement, where nutriment is not expended 

 by the due action of muscular force. The singular sucking-apparatus 

 upon the head of the Remorais an assemblage of peculiarly modified 

 and connected dermal spines. The more common modification is tlie 

 excessive development of one or more of the dermal spines, to form 

 peculiar weapons of defence. 



The Chimaeras, the Cestracions, and the Piked Dog-fish, show 

 such a stout bony spine, sometimes, as in the last-named shark, 

 sheathed with horn, at the front border of each dorsal fin, Avhich it 

 also serves to strengthen. The Fire-flares ( Trygon) and Eagle Rays 

 {Myliohates) have one or more strong, detached, barbed or serrated 

 spines, on the upper part of the tail. Agassiz has pointed out the close 

 resemblance of the microscopic structure of the bone of these spines 

 and the dentine of the teeth of the same kind of fishes : they are both 

 hardened by an outer layer of modified dentine, but as hard as 

 enamel. Many large fossil spines, called in Palaeontology " Ich- 

 thyodorulites," have been determined by their form and structure to 

 have belonged to extinct cartilaginous fishes, allied to the above-cited 

 existing genera, of which they are sometimes the sole indications 

 left by the wreck of former worlds. Amongst bony fishes, the 

 Siluroids (Sheat-fish) and Balistes (File-fish) are most remarkable for 

 these dermal weapons. In our rare Balistes capriscus the anterior 

 dorsal is sustained by three such spines ; the first much the strongest, 

 and the second subservient to the use of the first as a weapon, 

 rather than for the support of the fin. The first spine is articulated 

 by a very remai-kable joint to the broad interneural osseous plate : 

 its base is expanded and perforated, and a bony bolt passes freely 

 through the ring. Wlien this spine is raised, a depression at the 

 back part of its base receives a corresponding projection from the 

 contiguous base of the second ray, which fixes it like the hammer of 

 the gun-lock at full-cock, and it cannot be forced down till tlic suiall 

 spine has been depressed, as by pulling the trigger : it is then re- 

 ceived into a groove on tlie supporting plate, and offei's no impedi- 

 ment to the progress of the fish through the water. Tlie name of the 

 genus {Balistes^ and the connnon Italian name of the species in 

 question (Pesce balestra) refer to this structure : the spine of the 

 Balistes is also roughened with ganoid or enamel grains like a file, 

 whence our English name for it, ' File-fish.' The margins of the ana- 



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