192 BIVALVES. 



signal and is drawn up again to the surface. 

 Many and great are the dangers attendant on this 

 employment. The greedy shark often marks the 

 diver for his prey, his only chance of safety under 

 such circumstances, is to stir up the mud in the 

 water, and so elude the animal's observation. A 

 large flat fish also sometimes attacks him, and 

 keeps him under water till he is drowned. Even 

 when drawn up in safety to his vessel, he often 

 in consequence of his exertion discharges blood 

 from his mouth and nostrils. Most necessary 

 therefore is the regulation that the divers should 

 rest and labour alternately, during periods of 

 about ten minutes. 



Genus P I N N A* Plural, Pinna. 



SEA WING. 



Generic Character. Shell bivalve, equivalve, 

 fragile, thin, gaping at one end, and furnished 

 with a byssus ; shape J subtriangular, narrow at 

 the beaks, and expanding to a considerable 

 breadth at the opposite extremity ; hinge without 

 teeth ; valves united by a long external ligament. 



OBSERVATIONS ON THE SHELL AND ITS 

 INHABITANT. 



The Pinna seems to form the connecting link 

 * Plate X. figure 4. 



