Millimeters 



ro 



Figure 49. — Cross section of shell of an adult C. virginica 

 after decalcification in weak acid, Mallory triple stain. 

 Conchiolin of the prismatic layer is reddish-brown; that 

 of calcite-ostracum is bluish. 



oyster shell. In C. virginica, C. amjulata, and 

 many other species this area is highly pigmented; 

 in 0. edulis, C. gigas, pigmentation is either 

 absent or very light. 



The muscle scar in C. virginica is located in the 

 postero ventral quadrant of the shell (figs. 15, 21, 

 33). To a certain extent the shape of the scar 

 reflects the shape of the shell, being almost round 

 in broad and round oysters and elongated in 

 narrow and long shells. The area of scar is 

 slightly concave on the side facing the hinge and 

 conve.x on the opposite, i.e., ventral side. Curved 

 growth line, parallel to the curvature of the 

 ventral edge of the valve, can be seen on the 

 surface. They are most pronounced in the ventral 

 part of the muscle impression. Size and shape of 

 the scar is variable and often irregular (fig. 50). 

 The outlines of the impressions shown in this 



figure were obtained in the following manner: 

 the periphery of the impression was cu'cumscribed 

 with soft pencil; a piece of transparent Scotch 

 adhesive tape was pressed on the impression and 

 the outline was lifted and mounted on cross- 

 section paper; the area occupied by the impression 

 was measured by counting the number of squares. 

 Using this method, I obtained the replicas of 

 muscle impressions from 169 shells taken at 

 random from various oyster beds of the Atlantic 

 and Ciulf Coasts. The impressions are arbitrarily 

 arranged in four series (A-D) according to their 

 shape and size. The impression areas of round 

 and broad shells are shown in the two upper rows, 

 A and B; those of long and narrow shells are 

 arranged in the two lower rows, C and D. 



It may be expected that the larger is the shell 

 the greater is the area of muscle impression. 

 The relationship, as can be seen in fig. 51, is 

 rectilinear although the scatter of plotted data 

 is considerable and the variability increases with 

 the increase in size. The ratio of nmscle impres- 

 sion area to sliell surface area varies from S to 32 

 with the peak of frequency distribution at 16 to 

 18 (fig. 52). 



A small oval and unpigmented area on the 



C3?e7 o c? cp 







6 o 



Centimeters 



Figure 50. — Variations in .shape and size of muscle scars 

 on the shells of C. virginica. Rows A and B show the 

 types of scars normally found on broad and rounded 

 shells, the length of which is almost equal to or exceeds 

 the height. Rows C and D are the scars often found 

 on long and narrow shells in which the height exceeds 

 the length. Replicas of scars were made from shells 

 collected at random. 



42 



FISH AND WILDLIFE SERVICE 



