54 
ACANTHOPTERYGII. 
flexible one ; a large portion of the 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, 
