FISHES. 261 



posterior or narrow extremity is supported by an ef- 

 fective bony prop, fixed to the dorsal vertebrae. The 

 first spinous ray of the dorsal fin is attached to this 

 osseous platform by a broad hinge-like joint, which 

 permits motion only in the directions backward and 

 forward. The base of the second ray, which is also 

 thick, but flattened laterally, curves forward, and its 

 convexity, which is very rough, is opposed to a deep 

 groove on the back of the first. The root of the second 

 ray is furcated, and strides across the convexity of a 

 semilunar process, rising from the hollowed surface of 

 the bony platform, and on which it moves only in di- 

 rections parallel to the axis of the fin. The function 

 of this apparatus is as follows. When the first spinous 

 ray of the fin is erected, (either artificially or by the 

 muscular power of the fish.) the second follows, and 

 the rough convex process of its base is inserted within 

 and beneath the grooved surface of the first ; so that 

 the latter rests upon it, and is supported by it, with an 

 obstinacy proportionate to the depressing force ap- 

 plied ; while the direction in which the first ray presses 

 upon the base of the second is at the same time well 

 calculated to retain the second ray in its firm and erect 

 position. If the second ray be gradually depressed, 

 the first falls with it ; but both are as firmly fixed upon 

 ach other at every stage of their descent as when they 

 are erect, and upon the same principles. Some species 

 of Balistes have three spinous rays in the anterior 

 dorsal fin ; but the third ray takes no part in the 

 economy I have just described. 



Unlike most other fish, the Balistes employ their 

 tail but little in swimming a rapid lateral action of 

 their long dorsal and anal fins being their principal 

 mode of progression. 



