Shell morphology: The shell is large, up to 120 mm long, modioliform, thin and fragile. Essentially equivalve, the shell is elliptical in immature specimens, becoming increasingly arcuate in the larger, older specimens. The anterior margin is narrowly rounded, whilst the posterior margin is broadly rounded. The ventral margin is straight to slightly concave in small specimens, and becomes increasingly concave in larger specimens. The dorsal margin is convex. The umbones are often eroded, prosogyrate and nearly terminal to slightly subterminal. The prodissoconch I from 100 - 110 um in length. Prodissoconch II is reddish, between 385 - 404 um in length. An indistinct, raised, broadly rounded ridge or keel extends from the umbonal region to the postero-ventral margin. The external surface lacks any sculpture and is smooth except for concentric growth lines and fine radial periostracal corrugations in the median ventral area. The shell is dull-white in colour, beneath a dark-brown to straw-yellow periostracum. The antero-dorsal portion of the shell can be variably eroded depending on the age and collection site. The interior of the shell is off-white in colour, predominately nacreous. The ligament is opisthodetic, parivincular and extends posteriorly from the umbones to occupy 38 - 58% of the dorsal margin. The adult hinge is edentulous, thickened below and anterior to the umbo. Juvenile hinge has 8 - 9 denticles immediately posterior to the ligament and with 4 - 5 denticles located immediately below the umbones. The hinge denticles are obsolete in specimens greater than 11 mm in length.
Muscle scars: The muscle scars and pallial line are indistinct. The anterior adductor scar is oblong, below and posterior to the umbo. The posterior adductor scar is round, and contiguous dorsally with a single posterior byssal-pedal retractor scar. The anterior retractor scar is located in the posterior portion of the umbonal cavity. The posterior byssal retractors form a continuous scar that extends from directly beneath the posterior end of the ligament to the antero-dorsal edge of the posterior adductor scar. The ventral pallial line is straight and well inset, paralleling the ventral shell margin and extends from the postero-ventral edge of the anterior adductor scar to the posterior adductor.
Musculature: Although the posterior byssal retractors form one continuous scar in each valve, the posterior byssal retractors are subdivided into six main muscle bundles. The posterior pedal retractors are small, thin and arise from the antero-dorsal portion of the foot mass and pass lateral to the anterior bundle of posterior byssal retractors. Siphonal retractors are not evident. The anterior retractors arise just ventral to the origin of the anterior portion of the posterior byssal retractors on the dorsal surface of the byssal-pedal mass, and pass anteriorly to insert in the posterior portion of the umbonal cavity. The paired labial palp suspensors are slender and extend forward as branches of the anterior retractors to attach just behind and adjacent to the anterior adductor. The posterior adductor is oblong, whilst the posterior adductor is small and round in cross-section.
Foot & byssus: The foot is thick, with a shape that varies in preserved specimens depending on the degree of contraction. The byssal strands and light to dark-brown in colour, and are wide, flat and unornamented. The byssal gland extends down the foot behind the byssal groove.
Mantle & mantle cavity: The connections between the edge of the ascending lamellae and the surface of the mantle lobes and the visceral mass are weak or lacking, resulting in an incomplete separation of the incurrent and excurrent chambers. The muscular longitudinal ridges for the attachment of the ascending lamellae to the mantel lobes and visceral mass are lacking. The ventral edges of the inner mantle lobes are thickened and muscular. In live specimens the excurrent tubuliform siphon is capable of moderate extension beyond the edge of the shell. The horizontal branchial septum between the incurrent and excurrent chambers is lacking. The incurrent and excurrent chambers are not physically separated posterior to the posterior adductor. The posterior end of the gill axes attach to the inner wall of the fused inner mantle lobes just ventral to the excurrent siphon. A short valvular siphonal membrane joins the right and left lobes, and extends anteriorly a short distance into the pedal gape. The anterior edge of the valvular siphonal membrane is smooth, and lacking central papilla. The pedo-byssal gape is extensive. The incurrent aperture extends from the anterior end of the valvular siphonal membrane to the posterior edge of the anterior adductor.
Ctenidia: The demibranchs are ca. equal-sized, tall and fleshy. The ventral margins have well-developed food groves. The dorsal food grooves are in deep folds just below the junction between the lamellae and both the mantle lobes and the visceral mass. The ctenidia is off-white in colour. The filaments are thin for Bathymodiolus, but broader than is typical of mytilids. Distal interlamellar junctions are lacking. The descending and ascending portion of each filament is connected apically to one-quarter the height of the demibranch. Every second to sixth filament is a 'principal filament' exhibiting short interlamellar septum that extends dorsally to a slight degree. 'Tubular connections' between the free edges of the ascending lamellae and the gill axes are lacking.
Labial palps: The paired labial palps are greatly modified from the typical filter-feeding type, appearing to function as a sorting area for material gathered by the foot rather than the ctenidia. The base of the inner and outer palp pair are widely separated. The inner pair is placed farther posteriorly, and are large and muscular with a long proboscid-like extension of the far posterior/ end. The mouth is situated at the basal mid-point of the anterior end of the inner pair of labial palps, farther posterior than is typical for mytilids. The outer pair of palps is more anterior, triangular, and muscular, but smaller than the inner pair with a shorter proboscid-like extension.
Digestive system: The stomach and direct intestine are situated on the left of the mid-line. The rectum is located on the mid-line posterior to the entry into the ventricle. The intestine leaves the posterior end of the stomach and transverse a short distance posteriorly to the left of the mid-line. The intestine/ rectum has a very short recurrent loop, which turns in a clockwise direction when viewed dorsally, just prior to turning upwards to enter the ventral aspect of the ventricle just posterior to the auricular ostia.
(Gustafson et al., 1998)
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