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General Description

Diagnosis: Length up to 220 mm, relatively thin shelled and elongate. In adult specimens The height/ length ratio of the valves are between 0.37-0.52, whilst the width to length ratio is 0.11-0.15. The dorsal and ventral margins are nearly straight and subparallel to each other, with the valves somewhat depressed in the central segment. The periostracum is glossy, ochre coloured with close-set commarginal striae and elevated lamellae in the anterior and middle sectors of the shell. A prosogyrate low umbo is situated at the anterior 15-28% of the valve length, with beaks that are not enrolled. A fibrous layer of ligament occupies 22-29% of the valve length and 50-73% of the posterior lamellar layer of the ligament.

Description: The size is medium sized and relatively thin shelled and elongate in outline, nearly equivalve. The periostracum is thin, glossy and ochre in colour, remaining at the periphery and on the posterior part of the shell. There are short, close-set commarginal elevated lamellae developed mostly in the anterior and middle sections of the shell. The sculpture of the shell consists of commarginal striae and inconspicuous radial lines that run from the umbo to the posterior margin. There is a very wide shallow depression running from the umbo to the middle of the ventral margin. The umbo is inequilateral, and is situated in the anterior 20% of the valve. The umbones are prosogyrate and low, with beaks that are not enrolled and not touching each other. The anterior-dorsal margin is hort and nearly straight. The anterior margin is rounded, whilst the ventral margin is nearly straight with a very slight depression in the middle. The posterior margin is broadly rounded, whilst the dorsal margin is nearly straight. The internal shell surface is white, with faint commarginal and radial striations. The pallial line is slightly impressed anteriorly and hardly discernable posteriorly. Secondary pallial sacrs are present in the anterior part of the pallial line. The pallial sinus is an irregular, very shallow indentation below the posterior adductor scar. The anterior adductor scar is ovately conical and somewhat impressed to the rear. The posterior adductor scar is larger, subcircular and hardly distinct. The anterior pedal retractor scar is deeply impressed and located above the anterior retractor scar, but not fused with it. The posterior pedal retractor is not impressed and is fused to the posterior adductor scar. The nymph is weak, with a smoothly sloping posterior end. There is a fibrous layer of ligament that occupies 25% of the valve length and 56% of the posterior lamellar layer. Dentition of the right valve: ventral cardinal (1), and subumbonal cardinal (3b), consisting only of a posterior ramus. Cardinal 1 is upright, strong and wedge-shaped, bluntly pointed with subtrigonal borders, radiating ventrally. 3b is lower, with a rather wide base. It is narrower at the top, with the anterior margin steeply sloping and the posterior margin smoothly sloping, radiating posteriorwards. There is no fusion between teeth 1 and 3b, with their ventral parts diverging at about 90°. Dentition of the left valve: subumbonal cardinal tooth with two rami (2a and 2b) and a nearly totally reduced posterodorsal cardinal tooth (4b). 2b-ramus is very stout and wedge-shaped, whilst the 2a-ramus is lower and thinner. Proximal parts of 2a are connected with the lateral surface of 2b by a low bridge, whilst the ventral parts diverge at slightly less than 90°.

Anatomy: The mantle lobes are thin, except at the margins. The outer fold is most prominent and deeply folded. It secretes a two-layered periostracum, the outer one secreted within the periostracal groove, and the inner layer within one of the pleats of the outer mantle fold. The middle fold is composed of two subfolds, with the external subfold usually thinner, but the relative thickness of the subfolds changes along the mantle margin. The internal subfold of the middle fold is papillate anteriorly at the level of the anterior adductor. The inner fold is nearly inconspicuous forming a low ridge. Along the mantle margin there is a vascularized thickening of the mantle lobe extending from the anterior to the posterior adductors as in all studied vesicomyids. The mantle fusion forms three pallial apertures, the pedal gape and two posterior siphonal openings. The pedal gape occupies the entire ventral edge from the middle of the frontal surface of the anterior adductor to the base of the inhalant siphon. The posterior part of the mantle, including the outer and external subfold of the middle mantle fold, forms a muscularized envelop around the siphons. The internal subfold of the middle mantle fold has tentacles and borders both inhalant and exhalant siphons. The margin of the inhalant aperture bears one row of small tentacles, with the number of rows of tentacles increasing in the region between the apertures at the ventral margin. The tentacles of the exhalent siphon are much shorter. Internally, the inhalant siphon has on its dorsal side a ventrally directed, triangular flap of vascularised tissue. The volume of this flap can change, and in some specimens it is so large that it nearly entirely closes the lumen of the siphon. The exhalant siphon has an internal, thin, transparent sleeve. Between the siphons there is a septum, which joins the dorsal edge of the descending lamellae of the ctenidia. The anterior and posterior adductor muscles are large, with the posterior muscle oval in section, and the anterior muscle more elongate. The foot is well developed, rugose and very muscular. A short (ca. one-sixth of the sole length) and shallow byssal groove lies along the mid-ventral line of the foot and leads to a byssal aperture located slightly anterior to the heel. The ctenidia is thick, non-plicate and comprises of an inner single demibranch with descending and ascending lamellae. The ascending lamellae are two-thirds the size of the descending lamellae. The filaments of the lamella are connected with each other by interfilamental junctions. From every filament of both the ascending and descending lamellae a septum runs towards the inside, filling the entire interlamellae space. About every fifth to seventh septum is united with the opposite one, connecting the ascending and descending lamellae. Every interlamellar septum is divided into separate cylindrical tubes at full width of the septum. The outer parts of the tubes are wider than the inner ones. The tubes have a bead-like appearance with irregular enlargements and constrictions either along the whole length or only in their inner parts. The inner parts of the tubes are united by a thin transparent sheet of tissue. The ventral margin of the demibranch has a very shallow food groove. The margins of the ctenidia are fused along its entire length. Anteriorly the filaments insert between the palps, laterally the gill is connected by its ctenidial axis with the visceral mass and by the dorsal margin of the ascending lamellae fused with each other. Posterior to the foot, the margin of the ascending lamellae fuse with each other. The labial palps are small. The outer palps consist of a thickened ridge that runs along the anterior most part of the ctenidia, whilst the inner palps are smaller. The mouth is small, rounded and located beneath the posterior face of the anterior adductor, opening into the oesophagus which enters the anterior part of the thin-walled small stomach. There is no style sac. The gut leaves the posterior part of the stomach runs straight and passes through the visceral mass to the pericardial cavity. The gut penetrates the ventricle and ends in an anus located on the posterior surface of the posterior adductor muscle. The pericardinal cavity is large and elongate with thin transparent walls. The ventricle is thick-walled, muscular and surrounds the rectum. The auricles are thin-walled, with well-developed pericardinal glands located on its surface, opening into the ventricle approximately in its posterior third. The kidneys are located between the pericardium and the posterior adductor muscle. There are large gonads embedded in the posterior-dorsal part of the visceral mass, adjacent to the digestive galnd and occupying the dorsal part of the foot.

(Krylova et al., 2010)

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