194 ANATOMY OF VERTEBRATES. 



the base receives a prominence from the next bony ray, which 

 fixes the spine in the erect position, as the hammer of a gun-lock 

 acts at full cock ; and the spine cannot be forced down till the 

 small spine or ' hammer ' has been depressed, as by pulling the 

 trigger. This mechanism may also be compared to the fixing 

 and unfixing of a bayonet. When the spine is unfixed and bent 

 down it is received into a groove on the supporting plate, and 

 offers no impediment to the progress of the fish through the water. 

 The generic name (Balistes) and the common Italian name of the 

 fish (Pesce halestra), refer to this structure : the spine is rough- 

 ened by ganoid grains, whence our English name of ' File-fish.' 

 The hind border of the analogous weapon of the Centriscus 

 humerosus and of most Sheat-fish is denticulated, so that they 

 infiict a ragged wound. In all such weaponed osseous fishes, the 

 base of the spine is modified for articulation with another bone. 

 In gristly fishes so armed the base of the spine is simple, smooth, 

 hollow, implanted deeply in the flesh and attached to ligament 

 and muscle. 



The great majority of such weapons found in a fossil state, 

 called ' ichthyodorulites,' show by their basal structure that the}^ 

 come from Plagiostomous fishes, and exemplify in a remarkable 

 manner the efficiency, beauty, and variety, of the ancient armoury 

 of that order. In some, the marginal serrations were themselves 

 denticulate (Edestes)} Certain Rays {Trygon) have si^ines with 

 both margins serrate.^ 



The series of side-scales perforated by the mucous duct in the 

 modern soft-scaled fishes are usually more or less ossified. In the 

 Eel tribe the lateral mucous ossicles are tubular and concealed by 

 the cpiderm. In the Sole and Plaice the mucous scale bones of 

 the lateral line are quite superficial. There are many circular 

 radiated ossicles scattered over the dark or u]iper side of the skin 

 of the Turbot. A row of small chcvron-sliajied dermal bones 

 extends along the median line of the belly of the Herring, and the 

 extremity of each lateral process, fig. 37, dli, is connected with 

 that of the long and slender vertebral rib, completing the inferior 

 arch, like a sternum and sternal ribs. The Dory has two rows of 

 thick osseous plates along the under part of the abdomen ; but 

 their superficial position indicates their essentially dermal charac- 

 ter. Parts analogous to a sternum arc thus supjilicd from the 

 cxoskeleton as tliey arc from tlie splauchnoskelcton in the Lam- 

 prey, fig. 1 1 ; but the true honiologucs of the sternum arc first 



' CLXXX. 11. 121, fig. 38. - III. 123. 



