342 NATURE-STUDY REVIEW [16:8— Nov., 1920 



"Mollusk" means soft-bodied, the significance of which term we 

 realize more fully when we examine the beautiful and smooth 

 mother-of-pearl with whcih the shell is lined, and whose origin has 

 already been indicated. No more beautiful pigments have ever 

 appeared in paintings than those represented in nacre, as this 

 portion is scientifically known. Here is the original of the well 

 known pearl blue shade, rich purples and lilacs, blushes rivaling 

 those of the rose, salmons and the purest of whites. Unfortunately 

 vividness in nacre color detracts from the commercial value of the 

 shell, as the public desires mostly white buttons. Yet colored 

 buttons could be equally serviceable on less conspicuous garments, 

 and the great piles of wasted shells often to be seen along river 

 banks better utilized. The war has taught us that fads may be 

 utilitarian as well as otherwise. 



When minute, irritating objects such as small worms or grains of 

 sand find their way into the sensitive soft parts, they are in time 

 interred in that elegant mausoleum we term a pearl, which is 

 nothing more than concentric layers of crystalline calcium car- 

 bonate, (lime) deposited upon a nucleus of the type mentioned. 

 The thicker shelled animals seem to produce the best pearls, and 

 while there may be all shapes of them such as slugs, baroques, etc., 

 those which are most nearly rounded off are the most highly prized. 

 It has been thought that the shape of these depends on the relative 

 freedom of motion encountered while developing. Salt water 

 pearls are the most valuable, and it is interesting to know that both 

 kinds may be produced at will in an animal — an accomplishment 

 not every one is versed in. Frequently the pearls are tinted with a 

 suggestion of the color of the nacre or of the soft parts, and this 

 may either enhance or detract from their value. There is now 

 reason to believe that the coloration of both is related directly to 

 the chemical content of the surrounding water, while the latter in 

 turn may be associated with geographical conditions since the 

 brighter colors in some shells are found up stream. Similar condi- 

 tions apparently affect the outer covering, or epidermis of the shell. 

 Pearls are of value because good ones are scarce, and conditions 

 are unfortunate for the mussel fauna of a region where "pearl 

 fever" is rampant. Not only have thousands of animals been use- 

 lessly killed in the quest for pearls, but their valuable shells useful 

 in the button making industry, have been wasted. Worse than 

 that, natural propagation among them was terminated by taking 



