306 



POPULAR CONCHOLOGT. 



within and around the adductor muscle, and are said to be 

 occasioned by the overcharge of those glands whose func- 

 tion it is to secrete the nacrous fluid destined for the 

 internal lining of the shell. When the animal is thus 

 diseased, this beautiful iridescent fluid is very irregularly 

 discharged, being also deposited upon the inner surface of 

 the shell in little excrescences ; these are often detached, 

 and form articles of commerce as pearls of inferior value, 

 the former being considered more precious, both on 

 account of their rotundity of form and the clearness and 

 beauty of their complexion."* 



Excellent imitations of pearls are made with hollow 

 glass globules, the inside of which is covered with a 

 liquid called pearl essence, and then filled with white 

 wax ; the essence is composed of the silver-coloured par- 

 ticles which adhere to the scales of the Bleak fish, and 

 was first applied to this purpose, by a Frenchman named 

 Jacquin, early in the last century. The accounts of the 

 pearl fishery are exceedingly interesting. 



Pterinea. Goldfuss. — Shell e qui valve, transverse, 

 with a long straight hinge edge on both sides, prolonged 

 into an ear or wing, and on which the single ligament is 

 fastened ; there is also a notch for a byssus ; hinge with 

 teeth which diverge obliquely backwards, two muscular 

 impressions, of which the hinder and larger one is irre- 

 gularly three-sided, and placed entirely under the hind 

 wing. — Fossil. 



Vulsella. Lam. — Shell longitudinal, tongue-shaped, 

 nearly equivalve, irregular; bosses equal; hinge with a 

 prominent callosity in each valve, having an impression, 

 or arched pit for the ligament, wdiich is also partly ex- 



* Reeve's Conch. Syst. 



