Comprehensive Description

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General Description

The shell is thick, solid, short modioliform, inflated, equivalve and inequilateral. The shell height increases posteriorly, and the shell exhibits a somewhat trapezoid outline with a valve length/ height ratio of 044-0.58, and a width/ length ratio of 0.36-0.46. The dorsal margin is straight, whilst the ventral margin is weakly concave. The umbones are low, and are situated very slightly behind the anterior tip of the shell, except in some specimens where they are level with the anterior tip. A blunt and rounded ridge runs from the umbo to the postero-ventral corner, rather acute initially, but becoming flush posteriorly. The shell surface is opaque or pinkish, and is roughened with commarginal growth lines. A pale brown to dark brown periostracum covers the shell surface, but is occasionally eroded, especially around the umbonal region. The surface is hirsute, with the hairs triangular in shape and remaining only on the posterior region in fully grown specimens. In young specimens the hairs are long and sparse, but are present on the entire shell surface.

The internal surface of the shell is white in colour, and has a weak iridescence. The hinge plate is slightly thickened, but totally edentulous. The ligament extends from below the umbo almost to the halfway point of the dorsal margin. The anterior adductor muscle scar is spindle-shaped, and is located in a low (ventralward) position at the antero-posterior corner. The posterior adductor scar is oval and has an elongation from the fusion with the scar of the posterior byssal pedal retractor. The pallial line is unbroken.

The animal is milky white in colour. The ctenidia are large, rather thin and are fragile. They extend to four-fifths of the shell length, and about half of the shell height. The outer and inner demibranchs are almost equal in size and length. The mantle lobe is separated from the position of the anterior adductor to the postero-ventral corner. The mantle lobe is smooth and lacks frills. The valvular siphonal membrane is short and thin, and is without a papilla. The foot is cylindrical. The foot-byssus retractor complex has a moderate anterior byssal pedal retractor attached within the umbonal cavity well posterior to the umbo. The posterior pedal retractor and the anterior portion of the byssal pedal retractor are attached below the posterior end of the ligament. The attachment of the posterior portion of the byssal pedal retractor is elongated, and reaches the anterior rim of the posterior adductor muscle. The labial palps are triangular in shape; the posterior rami are large with a smooth surface, while the anterior rami are vestigial. The gut is straight.

(Okutani et al., 2004)


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