1943) are not supported by evidence. The inner 

 surface of bivalve shells may become slightly 

 eroded due to the increased acidity of shell liquor 

 when the mollusk remains closed for a long time, 

 but the erosion is, however, not localized; it 

 occurs over the entire shell surface. As to the 

 effect of the abundance of lime in the substratum 

 on the formation of chalky deposits, one must 

 remember that the concentration of calcium salts 

 dissolved in sea water is fairly uniform and that 

 calcium used for building of shells is taken 

 directly from the solution (see p. 103). Under 

 these conditions the abundance of calcium car- 

 bonates in bottom deposits cannot have any 

 effect on the formation of shell. 



Chalky areas of shell do not remain unchanged. 

 They become covered by hard substance and in 

 this way they are incorporated in the thickness 

 of the valves (fig. 41). 



Korringa's theory (1951) that the oyster 

 deposits chalky material ". . . when growing 

 older, in its efforts to maintain its efficiency in 

 functioning" and that ". . . where possible the 

 oyster always uses soft porous deposits when 

 cjuite a lot of shell volume has to be produced . . ." 

 is based on the assumptions: (1) that chalky 

 deposits most frequently develop in the area 

 posterior to the muscle attachment, (2) that the 

 layers of chalky material are more numerous in 

 cupped than in flat oysters, (3) that in the area 

 of the exhalant chamber (in the postero ventral 

 quadrant of the shell) the oyster attempts to 

 decrease the distance between the two valves by 

 rapid deposition of shell material, and (4) that 

 chalky material is used by the oyster "as a measure 

 of economy, as a cheap padding in smoothing out 

 the shell's interior." The validity of these 



assumptions with reference to C. piniinica was 

 tested by studing the relative frequency of the 

 occurrence of challvy deposits on the left and 

 right valves and by estimating the extent of 

 these deposits in different parts of the valves. 

 Tlie collection of shells studied for this purpose 

 compi-ised several hundred adult specimens from 

 various oyster beds along the Atlantic and Gulf 

 coasts. For determining the distribution of 

 chalky areas the inner surface of the valves was 

 arbitrarily divided into four quadrants shown in 

 figure 42 and designated as follows: A — dorso- 

 posterior; B — dorsoanterior; C — ventroposterior; 

 and D — ventroanterior. The following five classes 

 corresponding to the degree of the development 

 of chalky deposits in each quadrant were 

 established: 



No deposits within the quadrant 



1 to 25 percent of the area covered with 



deposits 1 



26 to 50 percent of the area covered witli 



deposits 2 



51 to 75 percent of the area covered with 



deposits 3 



76 to 100 percent of the area covered with 



deposits 4 



With a little practice it was easy to select the 

 correct class by visual examination. The first 

 question was whether there is an.y difference in 

 the frequency of occurrence and extent of chalky 

 deposits on right and left valves. For this 

 purpose the entire surface of the valve was exam- 

 ined and classified. Chalky deposits were 

 found as often on the right as on the left valve 

 of C. inrginica. This is shown in table 3 which 

 suniniarizps tlie observations made on 472 shells 

 collected at random at oyster bottoms along the 



_L 



J_ 



_L 



Centimeters 



Figure 41. — Left valve of an old C. virginica cut along the principal axis of growth. Chalky areas on both sides of the 

 hypostracum (dark platform for the attachment of the adductor muscle) are enclosed in the thin layers of hard 

 crystalline material. Hinge on the right. Xatural size. 



34 



FISH AND WILDLIFE SERVICE 



