54 



ACANTHOPTERYGII. 



flexible one ; a large portion of tlie remainder 

 have the division indicated by a depression in 

 the margin, or a cleft more or less deep, though 

 the membrane is continuous. In some cases, as 

 already intimated, the first dorsal is represented 

 by a few detached spines, either quite destitute 

 of membrane, or each furnished with its own. 



DORSAL FIN OF PERCH. 



In some of the Gurnards one or more of the 

 spinous rays are greatly prolonged beyond the 

 membrane ; in the Dory the membrane is pro- 

 longed between the spines into lengthened 

 threads; in the Sword-fish, the Opah, and the 

 Gemmeous Dragonet, the anterior portion is 

 elevated like a sail ; while in the singular genus 

 Pteraclis, of the American seas, figured on the 

 opposite page, both the dorsal and the anal are 

 so immense as to give to the vertical outline 

 of this fish somewhat the form of a butterfly 

 with expanded wings. The Gurnards have the 

 pectorals unusually developed, so that some 

 foreign species can use them as organs of flight 

 through the air. Examples of this, in a less 

 degree, may be observed in our native species. 



