Ostrea heffordi Finlay, 1928 (Pl. 3. figs. 3-8)

• 1880 (non) Ostrea reniformis Sowerby. Hutton, p. 175.
• 1884 (non) Ostrea reniformis Sowerby. Hutton, p. 533.
• 1904 (non) Ostrea reniformis Sowerby. Hutton, p. 93.
• 1913 Ostrea tatei Suter (in part). Suter, p. 889.
• 1928 Ostrea hefferdi. Finlay, p. 265. nom. nor. for O. tatei Suter (in part).
• 1957 Ostrea heffordi Finlay. Powell, p. 78.

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Description: Shell shape regularly subtriangular, valves thin and delicate. Right upper valve flat, sometimes domed beneath hinge. Distally the upper valve bears dark, concentrically laminated prismatic scales. Three to six dark bands are sometimes present crossing the valve to the outer margin. Ligament raised, triangular and winged laterally. Umbos are blunt on young shells and usually absent on older shells. Muscle scar lunate with anteroventral region extending further forward and lying alongside ventral margin of valve rather than centrally; slightly excavated on posteroventral margin. Colour of internal surface generally white, sometimes pink or yellowish-green; a conspicuous blue or purple band extends around distal posterior free margin.

Left lower valve is the valve by which the oyster is attached to the rock and the attachment scar occupies one half of the total area of the valve. Distally from the region of attachment, the valves turn upwards so that shell becomes either erect or lies at an angle to the rock. The distal free area is smooth with occasional low radiating ribs extending to the margin, ribs never projected into frills: distal shell margin moderately crenulated. External surface of left valve is grey to bluish-purple, occasionally pale brown. Internally the ligament is slightly raised, triangular in shape and winged laterally. Muscle scar identical in position to that of right valve, but not always the same size. Internally the free margins have a broad bluish-purple band, slightly wider than in upper valve. A few shells examined had no marginal colouration at all. Denticles occur regularly, there being between one and eight placed on either side of valve just posterior to the hinge; pits corresponding to the denticles are present on the upper valve.

Type: Suter's type (Ostrea tatei) and Finlay's neotype (Ostrea heffordi) are both held at the Geological Survey, Lower Hutt.

Distribution: Ostrea heffordi is known from the rocks at Dunedin and Lyttelton.

Vernacular Names: Dunedin rock-oyster; Southern rock-oyster.