102 PALAEONTOLOGY 



depth. If fishes of cognate habits with the present deep-sea 

 fishes, under whatever difference of form such Silurian fishes 

 may have been manifested, had really existed, we might 

 reasonably expect to find the remains of some of the countless 

 generations that succeeded each other during that vast and 

 indefinite period, sufficing for the gradual deposition of sedi- 

 mentary beds of thousands of feet in depth or vertical thickness. 



The evidences of plagiostomous fishes afforded by fossil 

 spines will be here pursued. In most of the existing cartila- 

 ginous fishes of this order the defensive spine which stands 

 erect in front of the dorsal fin is smooth ; such is the case in 

 the dog-fishes (Spinacidce) in which each dorsal fin is fronted 

 with a spine. In the Port-Jackson sharks (Cestraciontidce) 

 the spine in front of each dorsal is bony, and is armed along 

 its hinder or concave border with bent spines. The fin is 

 connected with this border, and its movements are regulated 

 by the elevation or depression of the spine during the peculiar 

 rotatory action of the body of the shark. This action of the 

 spine in raising and depressing the fin, resembles, Dr. Buckland 

 has remarked, that of the moveable or jointed mast, raising 

 and lowering backwards the sail of a barge. But their more 

 obvious use, in the small Plagiostomes possessing such spines, 

 is as defensive weapons against the larger and stronger voracious 

 fishes. The spine of the Onchus indicates its danger from some 

 larger, and as yet unknown, predatory fish. 



Certain bony fishes are similarly armed — c. g., sticklebacks 

 (Gasterostci), sheat-fishes {Sihtridce), trigger-fishes {Batistes), 

 and some species of snipe-fishes (JFistv£arid<&\. In the latter 

 family the Ccntriscus humcrosns (fig. 23) shows a dorsal spine, 

 denticulated behind, as in the Cestracionts ; but the base of 

 the spine in bony fishes is peculiarly modified for articulation 

 with another bone. In fche Plagiostomes the base of the spine 

 is hollow, becomes thin and smooth when the body of the spine 

 is sculptured, and is in the recenl fish implanted in the flesh. 



