122 PALEONTOLOGY. 



the spine in front of eacli 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. Buck- 

 iand has remarked, that of the movable 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. 



Certain bony fishes are similarly armed — e.g., stickle-backs 



Fig. 35. 

 Centriscus Jtumerosas. 



(Gasterostei), sheat-fishes (Sihtridce), trigger-fishes (Batistes), 

 and some species of snipe-fishes (Fistularidce). In the latter 

 family the Centriscus humerosus (fig. 35) shews 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 the Plagiostomes the base of the spine 

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

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

 The following genera of plagiostomous fishes have been 

 founded on the fossil spines, or, " ichthyodorulites," * which 



* Ichthys, a fish ; dora, a spear ; lithos, a stone. 



