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Figure 91. — Small area of the mantle edge with the adjacent part of the newly secreted shell viewed from above. The 

 mantle was exposed by cutting off a piece of the opposite valve, and the oyster was placed in sea water under a 

 binocular microscope, con.sh. — conchiolin sheet; mn.e. — edge of the mantle; i.l. — inner lobe; n.sh. — new shell; 

 o.mn.l. — outer lobe of the mantle, contracted; p.os.g. — periostracal groove. Drawn from life. The position of 

 structures at the edge of the mantle in relation to one another: the new shell area (n.sh.) marked on three sides by a 

 brolien line is in the plane of the drawing, next to it is the outer lobe (o.mn.l.), then the conchiolin sheet (con.sh.), 

 the middle lobe (m.l.), and the inner lobe (i.l.) is at the top, nearest to the observer. 



viscous conchiolin (conch. sh.) which oozes out 

 from the periostracal groove (p.os.g.) between the 

 outer and middle lobes. The distal edge of the 

 conchiolin sheet (end of stippled area) indicates 

 the previous maximal extension of the outer lobe 

 before the withdrawal of the mantle edge. The 

 entire group rests on the newly formed and already 

 solidified shell (n.sh.) . 



During the secretion of conchiolin the edge of 

 the mantle frecjuentlj' extends out and then with- 

 draws to the position recorded in the drawing. 

 At the time of expansion the outer lobe temporarily 

 supports the semiliquid conchiolin and by moving 

 in and out spreads it over the shell. Because of 

 this action the pro.ximal part of the newly formed 

 valve receives a larger amount of conchiolin and 



92 



becomes thicker than the distal portion. When 

 secretion is interrupted, the conchiolin layers 

 become incorporated into the shell substance and 

 the concliiolin sheet as shown in figure 91 is no 

 longer visible. 



The rate of secretion of the new shell varies at 

 different parts of the mantle edge. Quantitative 

 data are lacking, but observations made during 

 the periods of more rapid growth in (\ virginica, 

 (May to June and October to November in New 

 England waters) show that the area of newly 

 formed shell is always largest at the ventral side 

 of the valves near the principal axis of growth 

 (fig. 92). 



The organic matrix of the shell can be produced 

 by the pallial epithelium at any place along the 



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