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General Description

The shell is up to 96 mm long, and is oblong-oval, rather thick and moderately tumid. The anterior margin is broadly rounded. The ventral margin is convex, but is straight in the middle part. The posterior margin is rounded, and is somewhat more narrowly rounded close to the horizontal midline. The umbones are very shallow. The beaks are prosogyrous, and are situated well in front of the vertical midline. In fully grown specimens the beaks are just before the end of the anterior third of the valve.

The shell surface has fine, dense, irregular commarginal ridges and some coarser irregular growth waves and stages, which are stronger and more pronounced towards the ventral part and close to the ventral margin. There is also some fine, irregular and more or less oblique wrinkles on the anterior and postero-dorsal part of the valves, partly becoming slightly laminar. The lunule and escutcheon are absent. In place of the escutcheon there is a sulcus parallel to the postero-dorsal margin with a rounded delimitation, which can be characterised as a second posterior angle. The posterior angle is closely below this ridge, and is rounded but well marked, ending at the posterior extremity. Between the two “angles” there is a shallow radial depression, in which the wrinkly sculpture is more pronounced than on the rest of the shell surface. The periostracum has a yellowish colour, and is glossy on the earlier part of the shell, becoming duller ventrally and posteriorly.

The hinge line is short and rather narrow but is strong. The right valve has a lower anterior cardinal (1) that is strong and laminar, starting almost parallel to the anterodorsal margin, then curving upwards and ending directly under the beaks. The posterior cardinal is longer and stronger, and is assumed to be a fused 3a and 3b. It starts above and parallel to the lower part of the anterior tooth (1). It ascends towards the umbo and then descends backwards to beneath the anterior part of the ligament, with the anterior limit almost vertical. The left valve has a long, strong and somewhat irregular tooth 2a anteriorly, which ascends towards the dorsal margin and is situated under the umbo and in front of it. Cardinal 2b is fused with cardinal 2a under the umbo, and descends to the ventral margin of the hinge plate and is inclined anteriorly. The laminar posterior cardinal (4b) is posterior to the teeth 2a and 2b. It is shorter than 2a and 2b, and is inclined towards the posterior. The ligament is external, and opisthodetic. It is rather short, and is on a narrow but strong nymphal plate. The subumbonal pit is very small, and is shallow to indistinct. The pallial sinus is short but well developed, and is nearly triangular in shape. It is acute and points to about the middle of the anterior adductor scar. There is no prolongation of the pallial line on the anterior side of the sinus extremity. The anterior pedal retractor scar is deep, and is separate from the anterior adductor scar. The posterior pedal retractor scar is united with the posterior adductor scar. The inside of the valves have very faint, irregularly sized and spaced vermiculations or waves, which are more pronounced in the posterior part. There is also a very weak rib that runs from the umbonal cavity to the lower tip of the anterior adductor scar.

The valves are entirely white in colour.

The ctenidia are large and fleshy. The inner demibranch is about half the shell length (53 mm in the holotype), while the outer demibranch is shorter, measuring 45 mm in the holotype. The food groove is very narrow and is visible in both demibranchs. The exhalant and inhalant siphonal tubes are small but rather long, and are fused over their whole length, with numerous very short tentacles around the common opening. The siphonal tentacles have a small thickening at their ends. The inner mantle fold has very small, blunt tentacles in its posterior part for a length of about 3 mm (holotype) on both sides, running from the mantle fusion under the inhalant siphonal tube towards anterior. The foot is muscular, and is somewhat laterally flattened.

(Cosel & Olu, 2008)


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