"CRIPPLED" SHELLS THAT YIELD PEARLS 161 



of the shell it is comparatively smooth. These condi- 

 tions indicate that the pearl was lost from the shell dur- 

 ing the sixth year of the mussel's life. As additional 

 evidence there is a mark on each shell at the same place 

 between the fifth and sixth winter rest lines. The marks 

 indicate that the mussel sustained a slight injury during 

 its sixth year. This was possibly caused by the momen- 

 tary intrusion of a dredge hook which may have assisted 

 in removing the pearl at the time it was pulled from the 

 shell. While the winter rest periods are not plainly 

 marked after the sixth year, it is probable that about five 

 more years elapsed before the mussel was taken from 

 the water, making the age of the mollusk eleven years. 



The two shells illustrated are about the same size, yet 

 the reader will observe that the distortion is much greater 

 in the valuable "cripple" than in the worthless one. 

 There is much variation in the "cripples" which are found 

 by pearl hunters, and it is a good plan to examine and 

 study all of them carefully. 



The irregular shapes of the crippled shells are probably 

 caused primarily by the presence of some parasite inside 

 of the shell which interferes with the normal secretions 

 of shell forming material. 



The mantle supplies material for the epidermis, the 

 middle portion of the shell and the nacreous lining. A 

 portion of the mantle may be wrapped around the para- 

 site, or otherwise removed from its proper position, 

 which would interfere with the normal secretion of one 

 or more of the three shell forming materials. It is probable 

 that a part of the material which should be used to build 



