Description

Overall valve form: Maximum valve length 20.2 mm, average length 17.4 mm, maximum valve width 19.7 mm, average 16.1 mm. Valve width approximately equal to valve length, sometimes more or less, W%L averaging 92.8% (range 80.0%−107.1%, SD 4.65). Valve thickness approximately half valve length, T%L averaging 55.7% (range 42.3%−67.1%, SD 3.40). Maximum valve thickness 11.5 mm, average 9.7 mm.

Overall valve shape sub-pentagonal to rounded, widest approximately two thirds valve length, dorsal valve more rounded with the widest point closer to midway (Figs 5 A, B, 6 E, 7 A, B). Valves equally biconvex, not greatly inflated (thickness averaging 56% of length as stated earlier). Lateral commissure straight but descending slightly towards ventral valve anteriorly (Fig. 5 C, D). Anterior commissure generally flat-topped uniplicate, though variable, fold occupying medial ~third of valve width, anterior edge folded towards the dorsal valve, the dorsal edge of this fold being truncated (flat-topped) (see Fig. 5 E–G), extent of folding various from subtle (Fig. 5 E) to more pronounced (Fig. 5 F); anterior commissure is either non-dentate (Fig. 5 E, F) or with some light dentition (or secondary plication) (Fig. 5 G); antero-medial groove on ventral valve (formed by uniplicate folding) shallow. On the type specimen the lateral and anterior commissure folding is comparable to TMAG material, straight along lateral commissure (Fig. 7 C, D), and with pronounced flat-topped uniplicate folding at anterior commissure (without dentition) (Fig. 7 E).

Valves impunctate, opaque or translucent on smaller specimens, colourless (cream or whitish). Fine to prominent growth striae present on smaller to moderately sized specimens (Fig. 5), older thickened specimens with surface roughened and striae obscured except some near anterior commissure where they are more pronounced (Fig. 7 C–E).

Dorsal valve interior: Cardinalia illustrated for TMAG specimen in Figure 6 A–D. Cardinal margin approximately 112−116°. Cardinal process essentially absent, a small medial depression at posterior umbo being the likely point of diductor muscle attachment (some thickening occurs at this area in the older type specimens; Fig. 7 F). Sockets with well-developed outer and inner socket ridges, each inner ridge forming an angled corner antero-laterally. Sockets with strong corrugation or internal ridges (matching corresponding grooves on ventral valve teeth). Outer hinge plates forming narrowly triangular and concave plates between each inner socket ridge and crural base. Inner hinge plates absent (leaving V-shaped cavity between crural bases). Crural bases diverge at approximately 34° on TMAG specimens, outer hinge plates attached along outer faces of crural bases (see Fig. 6 D); beyond hinge plate crura are relatively short ('free length’ ~40% the total length of the crura back to their point of origin near the umbo) and curve slightly medially. Crura ‘falciform’, laterally compressed, concave along their inner face, distally broad and truncated, the truncated edge being slightly serrated.

On the holotype and paratype material (Fig. 7 F–I), which represent older and thickened specimens, the crural bases have become greatly thickened and swollen, postero-medially coalescing, and laterally overgrowing the outer hinge plate (the plate forming a narrow groove between the swollen crural base and the inner socket ridge). On the type material the free crura were weathered away (likely as the specimens were already dead and/or disarticulated when collected), therefore Hedley (1905) only illustrating the crura as short points (see Fig. 1 A).

Median septum reduced to a fine and low ridge, extending less that a third the dorsal valve length (just visible in Fig. 6 A, C, D).

Ventral valve interior (see Fig. 6 E–H): Beak small, length (posterior to dorsal valve) 8%–9% of total valve length, straight–suberect. Beak ridges sub-mesothyridid (but not readily apparent). Foramen tiny and trans–sub apical. Deltidial plates small and medially conjunct as a symphytium, often with a medial seam still visible where the plates meet. Symphytium very short (in antero-posterior axis), yet relatively wide, forming a thin edge leading towards the hinge teeth, mostly exposed. Where they contact medially the deltidial plates fold ventrally to form a large and tubular collar running posteriorly to the foramen (below the deltidial plates) (see Fig. 6 G, H). Hinge teeth small, with dorsal faces ovoid–rectangular and bearing series of ridges (matching ridges or corrugations in the dorsal valve sockets). Teeth supported by dental plates (or lamellae) (see Fig. 6 G, H).

On holotype and paratype material (Fig. 7 G, J) the delthyrial area is similar, deltidial plates conjunct with medial seam visible (paratype Fig. 7 J) or obscured (holotype Fig. 7 G), with the beak ridges slightly more visible, and the hinge teeth more worn.

Soft tissue (illustrated in Figure 8): Pedicle short, not extending much beyond beak. Lophophore spirolophous, comprising a pair of elongate arms, each coiled into a spiral. When coiled the lophophore takes up only half the valve length, but when fully extended each arm (from where the crura attach) is approximately 2× the dorsal valve length. Muscle attachment fields reach ~35% ventral valve length from beak, ~40% the dorsal valve length (from posterior end).

Pallial sinuses on both ventral and dorsal valves have essentially the same pattern and comprise a pair of well-developed vascula media originating from the antero-lateral edges of the main coelomic cavity for each valve, as well as multiple fine ‘vasculae’ (ves) radiating from the coelomic cavity. On each valve the main stems of vascula media (vas.med) diverge anteriorly (their bases being closer together on the dorsal valve), each then branches into a main anterior stem (an.st) which heads antero-medially in an arcing path to the anterior valve edge. The first (most posterior) lateral stem given off by each vascula media (lat.st) branches into another anteriorly directed stem (an.st.2) and a posterior stem (post.st) arcing back almost as far as the hinge. Further anteriorly along the an.st a second lateral stem (lat.st.2) can be defined. Each of these stems give off shorter lateral stems which in turn branch again before they reach the valve edge.

Gonads comprise four small patches, 2 per valve (more apparent on ventral valve, see Fig. 8 A). On the ventral valve they extend ~33% the valve length from the beak, each comprising a branching structure with an ovoid outline, positioned lateral the adductor muscles.