Comprehensive Description

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General Description

The shell is thick, solid, elongate modioliform, inflated, equivalve and markedly inequilateral. The umbones are low and are frequently eroded, exhibiting a pearly layer, and are situated 10 to 20% posteriorly from the anterior tip of the shell. The anterior portion of the shell is low and narrow, but the shell becomes higher and wider posteriorly, with the widest point at the middle or slightly behind. The anterior margin is round. The ventral margin first ascends in the anterior part, but is distinctly concave at the middle, then descends to a roundly expanded posterior margin. A weak postero-dorsal angle is present in young specimens, but is round to smooth in fully grown specimens. The ventral concavity tends to become more pronounced with growth, and the degree of concavity varies between specimens. The postero-dorsal margin is weakly convex. The general profile is axe-shaped, with a valve height/length ratio of 0.32-0.43, and a width/ length ratio of 0.26-0.35. The widest point is situated around the middle of the shell. A round blunt ridge is apparent initially, running from the umbo to the postero-ventral corner, but becoming obsolete towards the end. The shell surface is covered by a shiny, blackish periostracum, bearing rough commarginal lines. The periostracum is usually varnished black, but is occasionally dark brownish or greenish, even yellowish, particularly around the posterior margin, and varies by specimen in accordance with growth. A fine and rather indistinct radial sculpture is occasionally present in the ventral area.

The internal surface of the shell is dull white with a weak pearly luster. The hinge plate is thin and edentulous. The ligament is thick, long and extends to two-thirds of the posterior dorsal margin. The nymphal area is rigid. The anterior adductor muscle scar is round, and situated below the antero-dorsal margin of the shell, anterior to the umbo. The posterior adductor muscle scar is oval, and is fused with the scar of the posterior portion of the posterior byssal-pedal retractor. The pallial line is entire.

The animal is an opaque white. The ctenidia are thick, long and extends from the position of the labial palps to the posterior end of the mantle cavity. The mantle lobe is thick, smooth with a non-papillated valvular siphonal membrane. The labial palps are triangular, but very small.

The foot-byssus retractor complex is composed of a long anterior byssal pedal retractor that is attached just below the umbo in the umbonal cavity, a short posterior pedal retractor, the moderate anterior portion of the posterior byssal pedal retractor, both under the midpoint of the ligament, and the bifurcated posterior portion of the posterior byssal-pedal retractor. The posterior ramus is located in front of the posterior adductor muscle, while the anterior ramus is situated at the midpoint between the anterior portion of the posterior byssal retractor and the posterior ramus of the posterior portion of the posterior byssal pedal retractor.

(Okutani et al., 2004)


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