25 



220 

 a. 



15 



UJ 



10 



< 5 



UJ 



2.0' 



< 

 o 



UJ 

 Q. 



O 



u 



1.5 



1.0 



CO 



o 

 a- 5 



MdL MEDIAN 



Q^ LOWER OUARTILE 



Qj UPPER OUARTILE 



i i 



VI VII 



MONTHS 



Figure 102. — Seasonal changes in the rate of deposition of shell material over the inner surfaces of left (lower) valv'es. 



the area occupied by a piece of plastic, and a shell 

 ridge was formed along the edge of the mantle, 

 which was withdrawn a considerable distance back 

 from its normal position. It can be deduced from 

 these observations that injury to the mantle stim- 

 ulates the shell secretion and that deposition may 



take place even at low temperatures when normal 

 shell growth is inhibited. This would indicate 

 that the cnzynmtic system involved in shell dep- 

 osition is always present and may become active 

 in res]3onse to pathological conditions in spite of 

 the inhibitory effect of winter temperatures. 



106 



FISH AND WILDLIFE SERVICE 



