Comprehensive Description

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General Description

Shell morphology: The shell is small, less than 15 mm long, and is modioliform, sturdy and stout. It is translucent and essentially equivalve. The anterior margin is sharply rounded, whilst the posterior margin is broadly rounded. The ventral margin is straight but with a concave indentation in the region of the byssal gape. This indentation is more pronounced in the longest specimens. The dorsal margin is broadly convex, and more or straight over the span of the ligament. The umbones are often eroded, prosogyrate and subterminal, and are positioned within the anterior one-twentieth. A raised, broadly rounded external ridge extends from the umbonal region to the posterior-ventral margin.

External sculpture is lacking, with the shell being smooth except for concentric growth lines. The colour of the shell is dull-white beneath a straw-yellow periostracum. The antero-dorsal portion of the periostracum is variably eroded, and the periostracum is sometimes lacking on the dorsal three-quarters. The interior of the shell is off-white and predominately nacreous.

The ligament is opisthodetic, parivincular and extends posteriorly from the umbones to occupy from 31-37% of the dorsal margin. The adult hinge is thickened below and anterior to the umbones, with 12-28 denticles immediately posterior to the ligament and 9-19 denticles immediately below the umbones on a thickened boss.

Muscle scars: The muscle scars and pallial line are indistinct. The anterior adductor scar is round and somewhat truncated posteriorly. It is positioned near the antero-ventral margin, below the umbo. The posterior adductor scar is round and contiguous with the posterior byssal-pedal retractor scar dorsally. The anterior byssal retractor scar is located within the upper extremity of the umbonal cavity directly beneath the umbo. There is an elongated posterior byssal-retractor scar that is not divided. It lies parallel to the antero-posterior axis of the shell, and the posterior end of the muscle scar borders the posterior adductor scar anterodorsally, whilst the anterior end terminates below the posterior hinge denticles. The ventral pallial line is straight without a dorsal concavity, and extends from the postero-ventral aspect of the anterior adductor scar to the poster-ventral edge of the posterior adductor.

Musculature: The posterior byssal retractor is continuous, and is not divided into posterior and anterior portions. It attaches to the shell from an antero-dorsal edge of the posterior adductor to just posterior of and below the ligament's posterior end. Separate pedal retractors are located medially, between the posterior byssal retractors and are partially obscured when viewed from a lateral aspect. The pedal retractors become integrated with the posterior byssal retractors at the point of shell attachment. The anterior retractors arise from the dorso-lateral aspects of the foot mass and extend anteriorly to attach to the shell in the antero-dorsal extremity of the umbonal cavity. The labial palp suspensors are not evident. The posterior adductor is rounded, whilst the anterior adductor is slightly oblong.

Foot and byssus: The foot is thick, with the shape variable is preserved specimens depending on the degree of contraction. Byssal strands are white to light-brown in colour, and are thin, flat and unornamented. A purple tinted byssal gland extends down the foot behind the byssal groove, and extends laterally and slightly dorsal to the origin of the anterior retractors.

Mantle and mantle cavity: The connections between the edge of the ascending lamellae and the surface of the mantle lobes and visceral mass is weak or lacking, resulting in an incomplete separation of the incurrent and excurrent chambers. Longitudinal muscular ridges for attachment of the ascending lamellae to the mantle lobes and visceral mass are lacking. The ventral edges of the inner mantle lobes are not thickened and muscular. The excurrent siphon is little more than a simple slit with a short extensible collar that is not capable of extension beyond the perimeter of the shell. There is an unusual internal diaphragm that occludes the ventral two-thirds of the excurrent siphonal opening. It is attached dorsally to a slender muscular bridge that connects the side walls of an internal opening approximately two-thirds of the distance from the siphon floor. Horizontal branchial septum are lacking, and the incurrent and excurrent chambers are not separated posterior of the posterior adductor. The posterior end of the gill axes attached to the inner wall of the fused inner mantle lobes just ventral to the exhalent siphon. A short valvular siphonal membrane joins the right and left mantle lobes, and extends anteriorly a short distance into the pedal gape. The anterior edge of the valvular siphonal membrane is smooth, lacking central papilla. The pedo-byssal gape is extensive. The incurrent5 aperture extends from the anterior end of the valvular siphonal membrane to the posterior edge of the anterior adductor.

Ctenidia: The lamellae are of unequal height. The ascending lamellae are two-thirds to three-quartes the height of the descending, resulting in inner and outer demibranchs forming short-armed W-shaped gills. The demibranchs are unequal. The outer demibranchs are shorter anteriorly, ca. 90-95% the length of the inner demibranchs. The status of the food grooves is not known due to poor preservation. The ctenidia is filamentous to moderately thickened, with the filaments off-white in colour. Distal interlamellar junctions are lacking. The lamellae are joined apically to ca. one-third the height of the gills. 'Principal filaments' and 'tubular connections' are lacking.

Labial palps: The paired labial palps are short, thickened and broadly triangular. The inner surfaces are plicate, whilst the outer surfaces are smooth. The bases of the inner and outer pair are coincident. Both pairs are in the normal anterior position, without proboscid-like extensions. The outer pair of pals is larger, up to twice the size of the inner pair. The mouth is situated normally, at the basal junction of the inner and outer palps.

Digestive system: The alimentary system is well developed for the group. The stomach and direct intestine are located slightly to the left of the body mid-line. The intestine leaves the posterior end of the stomach and passes posteriorly to the left of the mid-line and ventral to the ventricle. A very short recurrent loop to the right begins before the ventricle's mid-point. The recurrent intestine then passes beneath the ventricle to the right side of the mid-line and proceeds anteriorly for a short distance. The rectum then returns to the mid-line and enters the floor of the ventricle anterior to the auricular openings and about one-fifth of the distance from the ventricle's anterior.

(Gustafson et al., 1998).


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