BACK'S GRAYLING. 121 



is referable to it, in those other fishes which have 

 it so much, and often so curiously developed, the 

 power, when known, may probably be in the ratio 

 of its size. The colours of this grayling are beauti- 

 ful, but chaste; above of a lavender-purple, be- 

 neath greyish, with white spots; but the chief 

 adornment is the large fin, of a graceful curving 

 form ; it is of a blackish grey, but is relieved by 

 transverse rows of Berlin-blue spots. 



Chcetodon vespertilio will also exhibit great de- 

 velopement of this series of fins. It is not a very 

 uncommon fish, and has been many times figured. 

 There is scarcely any fishes which approach the 

 Pteraclis and this in the immense developement oi 

 the organs in question ; and, in the last, they givfl 

 to the fish collectively almost the appearance of 

 half-moon, of which the extremities of the dor-sa. 

 and anal fins, pointing backwards, constitute the 

 horns. The principal use of the dorsal and anal 

 fins of fishes, regarded merely as ministering to 

 locomotion, appears to be that of poising those 

 animals, and preventing them from continually 

 reeling over to one side during either their station 

 or progression in the water. The air-bladder, it is 

 true, in most fishes, running as it does along the 

 spine, tends to keep this part uppermost under 

 ordinary circumstances; but this provision would 

 have been insufficient to counteract the influence 

 of the waves and conflicting currents, without the 

 additional security afforded by the dorsal and anal 

 fins, which, by the salient angle which they form 



