Comprehensive Description

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General Description

The shell is medium-sized, with a valve length of up to 45 mm, stout, and is elongate-elliptical in outline, and equivalve. The periostracum is brownish in colour, and is nearly absent except for a meagre remnant present on the periphery of the ventral margin. The shell has a sculpture of irregular growth lines and wrinkles. The escutcheon is moderate, sometimes with a lunular incision developed. The shell is inequilateral, with an umbo situated at 27-31% from the anterior end of the valve. The umbones are prosogyrate, and are not very low. The beaks are slightly enrolled, but not touching each other.

The antero-dorsal margin is nearly straight or slightly convex, whilst the anterior margin is broadly rounded. The ventral margin is slightly convex, with the posterior margin from tapering to rounded. The postero-doral margin is nearly straight or slightly convex. The pallial line is somewhat broad, and not impressed. The anterior adductor scar is ovately conic, with the posterior margin of the anterior adductor scar slightly impressed to the rear. The anterior pedal retractor scar is shallow. The posterior adductor scar is slightly impressed anteriorly, square-rounded and fused to the posterior pedal retractor scar. The nymph is not strong, with a sloping posterior end. Along the posterior part of the base of the nymph there is a slight depression. A fibrous layer of ligament is attached to the nymph comprising of 62-71% of the posterior lamellar layer and 26-034% of the valve length.

Dentition of right valve: ventral cardinal (1) is stout, shelf-like and radiates obliquely anteriorwards. The 3a-ramus is lamellate, almost parallel to the anterior-dorsal shell margin, and is fused in its proximal part with the anterior edge of the 3b-ramus. The 3b-ramus is broad, with a slightly concave top surface and slightly rising anterior and posterior edges. The posterior lateral is obsolete, occupying a little more than half of the nymph length.

Dentition of the left valve: 2a-ramus is lamellate, parallel to the anterior-dorsal shell margin and fused in its proximal part with the anterior edge of the 2b-ramus. The 2b-ramus is broad and radiates ventral-wards. The 4b-tooth is elongate, low and occupies a little more than half of the nymph length.

(Krylova & Sahling, 2006)


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