Centimeters 



Figure 15. — Blue Point oyster (C. virginica) from Great South Bay, Long Island, N.Y. The size of this 5-year-old 

 oyster is about 10 x 6.6 cm. (4x3 inches). The shell is strong and rounded; its surface is moderately sculptured. 

 Left — outside surface of left valve. Right — inner surface of right valve. Small encircled area under the hinge on 

 the inner surface of right valve is an imprint of Quenstedt's muscle. 



Centimeters 



Figure 16. — Cross section below the hinge of an adult C. 

 virginica. Left valve at bottom, right valve at top of 

 the drawing. The buttress of the right valve fits the 

 depression on the left valve. The two valves are connect- 

 ed by a ligament (narrow band indicated by vortical 

 striations) which consists of a central part (resilium) 

 and two outer portions. Slightly magnified. r.V. — 

 right valve; bu. — buttress; dc. — depression or furrow on 

 left valve (l.v.); lig. — ligament. 



grown C. virginica the pointed end of the upper 

 (flat) valve is always shorter than that of its 

 opposite member (fig. 17). The angle between 

 the two beaks determines the greatest extent to 

 which the valves can open for feeding or respira- 

 tion and is, therefore, of significance to the oyster. 



If the oyster shell is oriented in such a way that 

 both of its valves are visible and the beaks point 

 up and toward the observer, the flat valve with a 

 shorter, convex resilifer is the right one and the 

 cuplike valve with the longer concave resilifer 

 is the left one. The dorsal margin of the oyster 

 is the beak or hinge side, the ventral margin the 

 opposite. If viewed from the right (flat) valve 

 with the hinge end pointing away from the ob- 

 server the anterior end of the oyster is at the 

 right side of the valves and the posterior is at the 

 left. 



The posterior and anterior parts of the oyster 

 shell may also be identified by the position of the 

 muscle impression, an oval-shaped and highly 

 pigmented area marking the attachment of the 

 adductor muscle on the inner side of each valve. 



MORPHOLOGY AND STRUCTURE OF SHELL 



17 



