Centimeters 



Figure 17. — Side view of a very old and large C. virginica from Stony Creek, Conn. Notice the curvature of the beak, 

 the depressed resilifer on the lower valve and the protruding rcsilifer on the upper one. The angle between the beaks 

 determines the maximum movement of the upper valve. Dimensions: height — 25.5 cm. (10 inches) and width^ — 6.4 

 cm. (2.5 inches). 



The muscle impression is asymmetrically located 

 closer to the posterior end of the valve. This 

 area of the attachment of the adductor muscle 

 has been called the "muscle scar." Some mala- 

 cologists prefer to use the expression "muscle 

 impression" or "area of attachment" (Stenzel, 

 personal communication) because the word "scar" 

 usually means the mark left by healing of an 

 injury. The proposed change in terminology 

 does not seem to be desirable because the name 

 "muscle scar" has been so well established in 

 scientific and popular writings that its abandon- 

 ment may cause confusion. 



The three principal dimensions of bivalves, in- 

 cluding oj-sters, are measured in the following 

 manner (fig. 20): height is the distance between 

 the umbo and the ventral valve margin; length is 

 the maximum distance between the anterior and 

 posterior margin measured parallel with the 

 hinge axis; and width is the greatest distance 

 between the outsides of the closed valves measured 

 at right angles to the place of shell commissure. 



In manj' popular and trade publications on 

 shellfish the word "length" is used instead of 

 "height", and tlie word "width" is employed to 

 designate the length of the oysters. To avoid 

 confusion the scientific rather than popular 

 terminology is used throughout the text of this 

 book. 



The shape of oyster shells and their proportions 

 are higlily variable and, therefore, are, in some 

 cases, of little use for the identification of species. 

 The variability is particularly great in the species 

 of edible oysters {C. virginica, C. gigas, C. angulata, 



and C. rhizophorae) that have a wide range of 

 distribution, thrive on various types of bottom, 

 and are tolerant to changes in salinity and turbidity 

 of water. Certain general relationships between 

 the shape of the oyster shell and the environment 

 are, however, apparent in C. virginica. Oj'sters 

 growing singly on firm bottom have a tendency 

 to develop round shells ornamented with radial 

 ridges and foliated processes (figs. 4, 15). Speci- 

 mens living on soft, muddy bottoms or those 

 which form clusters and reefs are, as a rule, 

 long, slender, and sparsely ornamented (figs. 

 19,21). 



The thickness and strength of the valves of 

 C virginica are higldy variable. Shells of oysters 

 grown under unfavorable conditions are often 

 thin and fragile (Galtsoff, Chipman, Engle, and 

 Calderwood, 1947). Likewise, so-called "coon" 

 oysters from overcrowded reefs in the Carolinas 

 and Georgia are, as a rule, narrow and have light 

 shells (fig. 19). Heavy and strong shells are not 

 typical for any particular latitude. They can be 

 found on hard, natural bottoms throughout the 

 entire range of distribution of C. virginica. I have 

 in my collection shells from Prince Edward Island, 

 Cape Cod, Delaware Bay, Louisiana, and Texas 

 which in shape and strength of valves are in- 

 distinguishable from one another. Sometimes 

 the growth of shells in length (in anteroposterior 

 direction) equals or exceeds the growth in height. 

 Such specimens, one from Texas and one from the 

 waters of Naushon Island off the Massachusetts 

 coast, were found in stickv mud. As can be seen 



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FISH AND WILDLIFE SERVICE 



