262 FUNCTIONS AND INSTINCTS. 



bed is rich the diver often collects 150 oysters 

 at one dip, but sometimes not more than five. 1 

 It is said that a single diver will, in one day, 

 often bring up from 1000 to 4000 oysters. 



From the simple circumstance that Provi- 

 dence has instructed this animal, which cannot 

 eject from its shell those substances, whether 

 formed within itself, or that have accidentally 

 entered, to encase them in the precious sub- 

 stance which it is empowered to secrete, what 

 a vast fund of ornament to deck the most lovely 

 part of the creation, and having no parallel in 

 any gem that the earth produces, is provided. 

 The pearls obtained from other shell-fish vary 

 in colour those from the wing-shell are brown, 

 and those from the fresh-water muscles greenish, 

 but sometimes they are yellow, pink, bluish, and 

 some are even black ; these last are very rare 

 and dear. 



Other bivalves fix themselves by a tendinous 

 ligament to the rocks. In one genus,* in the 

 upper valve near the hinge, is an aperture, 

 closed by a kind of operculum formed at the 

 dilated extremity of an internal muscle, it is by 

 this operculum that the animal fixes itself. In 

 another, related to the last, 3 the beak of the lower 

 valve turns up, overhanging in some degree 

 the upper valve ; in this beak is a notch or aper- 



1 Malte-Brun, Geogr. iii. 225. 2 Anomia, PL. V. FIG. 2, 3. 

 3 Terebratula, PL. V. FIG. 4. 



