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Comprehensive Description

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General Description

The shell is medium –sized, up to 60 mm long, and is moderately thick and solid. The shell is equivalve, and is triangular-modioliform, but the outline is variable. The valves are inflated, most so just in front of the vertical midline. The valve length/ height ratio is between 1.58-1.93. The umbones are rather small, terminal, and are anteriorly protruding. The beaks point antero-ventrally and are the anterior most part of the shell. The anterior margin is narrowly rounded or even almost angular, and is partly obscured by the beaks in an anterior view of the shell. The antero-ventral margin is more or less flaring towards the posterior, with the ventral margin being slightly concave to straight. The posterior margin is evenly rounded ventrally, and is markedly to slightly convex dorsally. The postero-dorsal corner is narrowly to broadly rounded, and is situated in front of the anterior margin of the posterior adductor scar where the highest part of the valve is situated. The postero-dorsal corner is ill-defined in some specimens.

The ligament plate is narrow, and is almost straight. The exterior of the shell has irregular commarginal growth lines and growth stages that are weakly reflected on the interior. There are a few very faint and more or less densely spaced radial striae that run from the beaks to the ventral margin near the byssus outlet. They are otherwise obsolete or missing, and are often only visible near the margin. In some adults there are faint, broad and very shallow radial undulations on the postero-dorsal slope parallel to the dorsal margin, and that end at the postero-dorsal margin.

The periostracum is thick, with a chestnut-brown to dark brown colour. Juveniles are lighter coloured. The periostracum is dull and has byssal bristles that are arranged commarginally on the posterior part except near the dorsal and antero-ventral margin, more or less dense or sparse to almost absent. The hairs are oblique, being more or less narrow with a broad base. The byssal endplates of other specimens may also be present on the shell surface, but are easily discernable from the byssal bristles.

The hinge is toothless. The ligament is opisthodetic, long and narrow, extending over about four-fifths of the postero-dorsal margin in front of the postero-dorsal corner. The ligament ends in a taper posteriorly. There is an almost obsolete subligamental shell ridge under the ligament towards the posterior, but beneath the umbones it may become a thin lamella which stretches towards and is ventrally parallel to the anterior margin. The anterior adductor scar is small and oval, and is situated close to the antero-ventral margin and posterior to the umbones. The posterior adductor scar is large and rounded, and is united with the scar of the posterior pedal and byssus retractor muscle complex, which is not divided into anterior and posterior scars. The anterior byssus retractor muscle scar is situated in the posterior end of the umbonal cavity, postero-dorsally to the beaks, and is visible only from a ventral perspective. The pallial line is almost straight to slightly concave.

The shell beneath the periostracum is a dull-whitish colour, and the shell interior is nacreous.

The ctenidia are long and narrow, measuring 44 mm long and 7.7 mm broad in a 59.5 mm specimen. The outer demibranch is shorter anteriorly than the inner demibranch. The ascending lamellae of the outer demibranch are anteriorly fused to the mantle for a very short distance (~ 1 mm), whilst those of the inner demibranch are fuse to the visceral mass for an equally short distance. The filaments are broad and fleshy.

The inner mantle folds are separate along the entire length of the ventral margin from the anterior adductor to the posterior margin. The mantle edges are narrow, thin and continuous over more than the anterior half of the shell length, then becoming broader, thicker and frilled towards the posterior. On the posterior 1.5-2 cm the mantle edges are more undulate or frilled and are more or less thick and fleshy. At the anterior end, the inner mantle folds continue over the ventral side of the anterior adductor, almost touching each other approaching and at their end, but with no backfold seen. The valvular siphonal membrane is short and thin, and has a thicker rim at the end but is without a median papilla.

The foot is medium-sized, measuring 17.5 mm long (including the byssus orifice) in a 59.5 mm specimen. The foot-byssus retractor muscle complex is short. The anterior retractor is rather short and broad, and is attached to the shell in the posterior part of the umbonal cavity, with the anterior most point being well behind the beaks. The posterior byssus retractor is composed of two long, broad and diverging muscle bundles that have a common base at the base of the byssus. The anterior part of the posterior byssus retractor consists of a broad bundle that is attached to the shell interior at about the middle of the valves, at an angle of about 50° to the longitudinal shell axis. The posterior part of the posterior byssus retractor consists of another very broad bundle that passes towards a long attachment point just in front of the posterior adductor scar. The posterior foot retractor is long and rather strong. It arises from the anterior side of the foot base behind the base of the anterior retractor muscles, and passes towards the anterior bundle of the posterior byssus retractor muscle, reaching the shell inside closely appressed to the bundle for a very short part of its length only.

The labial palps are large and long. The posterior pair is thick and fleshy. The posterior pair is thick and fleshy, whilst the anterior pair is smaller, measuring about 4.5 mm long in a 59.5 mm specimen. The palps of the posterior pair are longer and narrow-triangular in shape, measuring about 7 mm in a 59.5 mm specimen.

The stomach is situated just behind the middle of the anterior half of the shell length. It is elongate and simple, with a small anterior chamber. The posterior chamber is longer and slightly broader. Only four entrances of digestive diverticula were detected. The left pouch is small, and the intestinal groove is well visible. The midgut runs posteriorly straight and medially from the stomach to under the ventricle and enters it in front of the ostia, without any loop or curve. Behind the heart the intestine passes dorsally over the posterior adductor and towards ventrally on its posterior side. The anus is at the lower part of the posterior adductor, adjacent to the posterior end of the ctenidia.

The heart is rather narrow, and is situated in the posterior half of the shell length, with the anterior extremity of the ventricle at the mid-shell/mantle length. The auricles are long, and are fused posteriorly in front of the anterior part of the posterior adductor. They stretch out forward in narrow lobes to the middle of the body length, just in front of the anterior extremity of the ventricle and just in front of the anterior limit of the anterior muscle bundle of the posterior byssus retractor.

(Cosel & Janssen, 2008).

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