The shell is rounded trigonal in shape, heavy, inflated, and chalky grayish white. The umbones or “beak” (the oldest part of the shell, including the larval shell or prodissoconch) are slightly more anterior and prosogyrate over an impressed heart-shaped lunule. External sculpture consists of crowded, coarse commarginal growth lines that are elevated as erect lamellae anteriorly and posterorly, but lower and often smooth at the center of the valve. Posterior to the umbones, the smooth but distinct escutcheon slopes toward the posterior end. The ligament is external and extends from the lunule to approximately half the distance from the umbo to the posterior corner of the valves. The periostracum is thin and mostly nonpersistent in the adult. Structurally, the shell is aragonitic calcium carbonate, composed of three layers: an outer composite prismatic layer, a middle crossed lamellar layer, and an inner homogenous layer.
Internally the shell is porcellaneous white, usually flushed with dark purple near the margins. Minute denticles adorn the interior margin of the shell. The clam’s muscles attach to the shell and leave distinct scars on the inner surface. The two largest, the anterior and posterior adductor muscle scars, are oval to teardrop-shaped with the anterior and posterior pedal retractor muscle scars at their dorsal edges. The slightly impressed pallial line (caused by attachment of the mantle and/or pallial muscles to the shell) extends from each adductor muscle scar along the ventral shell margin. The siphonal retractor muscle (which retracts the short siphons of the clam) creates a triangular, anteriorly pointed pallial sinus, or embayment in the pallial line, near the posterior adductor muscle scar.
The strong hinge plate is heterodont, bearing three radiating cardinal teeth just below the umbo. Lateral teeth, in other groups paralleling the shell margin at a distance from the umbones, are absent in Mercenaria. A commonly used hinge formula, is still in widespread use, was developed by Bernard in 1895 based on the ontogenetic order of appearance of the teeth. In Mercenaria, the anterior, middle, and posterior cardinal teeth of the left valve are numbered 2a, 2b, and 4b, respectively. These interlock with the anterior, middle, and posterior cardinal teeth of the right valve, numbered 3a, 1, and 3b, respectively. In Mercenaria, the left middle, right middle, and right posterior cardinal teeth are bifid or radially grooved. Paralleling the elongated posterior cardinal tooth in each valve is the nymph or attachment area for the ligament.
Inset shows a free pearl formed by M. mercenaria.