78 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



most valuable being those which are round or pear-shaped, slightly 

 transparent, free from specks or blemishes, and possessing to the 

 highest degree the characteristic luster. 



Pearls are liable to deteriorate with age, contact with acids, gases, 

 and vapors, and though various methods are in use for restoring 

 them to their original beauty, they are by no means to be relied 

 upon, so that care should be taken to preserve fine pearls by wiping 

 them after use with a clean, soft cloth and keeping them wrapped 

 and in a closed box. 



Although nearly all bivalves with nacreous shells occasionally 

 yield pearls, practically all of those of commerce are obtained from 

 only a few families of mollusks, prominent among which are the 

 Aviculidae, Unionidae, and Mytilidae. The pearl oyster of the 

 Pacific and Indian oceans, which has yielded the bulk of the pearls 

 of the world, belongs to the first of these groups. The unio, or fresh- 

 water mussel, so abundant in the rivers and lakes of North America, 

 belongs to the second. 



The name baroque is given to irregular and sometimes almost 

 grotesquely contorted forms, formerly considered of little value, but 

 now when of good color highly prized. A series of these from fresh- 

 water streams in the Mississippi Valley is shown in figure 8. 



LIST OP SPECIMENS. 

 INDIA. 



Necklace of 148 round pearls; two pear-shaped pearls; white. Gift of the 

 Imam of Muscat Nos. 1846, ]847 



JAPAN (AGO BAY). 



Two culture pearls, one white, one pink; 7.5 and 3.9 grains; 7 and 5 mm. 



diameter. Gift of T. Kume No. 1497 



Culture pearl in shell. Showing growth of culture pearl. Gift of T. Kume.. No. 1498 



PANAMA. 



One lot of seed pearls. Gift of H. P. Petersen No. 1500 



UNITED STATES. 

 American fresh waters. 



Five turned pearls, white; total weight, 61.7 grains; average size, 10 by 

 6 mm. Isaac Lea collection No. 1495 



Fourteen wing-shaped; white, pale pink, pale brown; total weight, 117.2 

 grains. Isaac Lea collection No. 1494 



Forty-nine various shapes; white, pale pink, pale brown; total weight, 



321.8 grains. Isaac Lea collection No. 1493 



Arkansas. 

 Black River: 



Three white pearls, 48, 38, and 34 grains. Isaac Lea collection No. 1842 



Two pink pearls, 38.4 and 14.6 grains. Isaac Lea collection No. 1841 



