Diagnosis: Length to 277.4 mm, thin-shelled and strongly elongate. In adult specimens the height/ length ratio is between 0.21-0.32, whilst the width/ length ratio is 0.08-0.10. The dorsal margin is slightly convex, whilst the ventral margin is concave. The shell widens towards the posterior third, with a light brownish periostracum with close-set commarginal striae and elevated lamellae at the periphery. The umbones are low and prosgyrate, situated at the anterior 9-15% of the valve length. A fibrous layer of ligament occupies 26-40% of the valve length, and 55-69% of the posterior lamellar layer of the ligament.
Description: The shell is large and relatively thin-shelled, slightly elongate in outline and nearly equivalve. The periostracum is thin and brownish in colour, persistent mainly on the ventral posterior area, with short commarginal elevated lamellae. The shell sculpture consists of commarginal striae and inconspicuous radial lines running from the umbo to the posterior margin. There is a very shallow but broad depression running from the umbo to the middle ventral margin. The shell is inequilateral, with the umbones situated in the anterior 15% of the valves. The umbones are prosogyrate and low. The beaks are pointed forward. The anterior-dorsal margin is short and convex. The anterior margin is rounded, whilst the ventral margin is concave. The posterior margin is slightly tapering, and the posterior-dorsal is straight proximally and convex distally, with an obtuse angle between these two parts. The shell widens towards its posterior third, where the maximum shell height is reached. The internal shell surface is white, with faint commarginal and radial striations. A row of small pallial scars run from the umbonal cavity to the posterior pedal retractor scar. The pallial line is distinct and anteriorly impressed. The pallial sinus ia an irregular, very shallow indentation below the posterior adductor scar. Well-developed secondary pallial scars are present in the anterior part of the pallial line near the anterior adductor scar. The anterior adductor scar is ovately conical, somewhat impressed to the rear. The posterior adductor scar is larger, suboval and well impressed at its anterior limit. The anterior pedal retractor scar is deeply impressed and located above the anterior adductor scar, but is not fused with it. The posterior pedal retractor is slightly impressed and fused to the posterior adductor scar. The nymph is weak, with a smoothly sloping posterior end. A fibrous layer of ligament occupies 34% of the valve length and 60% of the posterior lamellar layer.
Dentition of the right valve: ventral cardinal (1) and subumbonal cardinal, consisting only of a posterior ramus (3b). Cardinal 1 is strong, wedge-shaped and pointed, radiating ventrally. 3b is lower, thinner and radiates posteriorwards. There is no fusion between 1 and 3b, with their ventral portions radiating at <90°. Dentition of left valve is as follows: subumbonal cardinal with two rami (2a and 2b), with a posterodorsal cardinal (4a) absent. The 2b-ramus is very short, wedge-shaped and upright, radiating posteriorwards. The 2a-ramus is lower and thinner. The proximal part of 2a in connected with the lateral surface of 2b by a low bridge, with the ventral parts diverging at <90°.
Anatomy: Abyssogena novacula shows all the characters of the genus. The interlamellar septum of gills consists of more or less rectangular tubes. The margin of the inhalant aperture bears two rows of tentacles, the outer one consisting of larger and sometimes branching tentacles, whilst those of the inner cycle are smaller. The margin of the exhalant siphon bears one row of quite small tentacles.
(Krylova et al., 2010)
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