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(Pl. XXV., fig. 11.)
Testa tenuis, magna, compressa, rotundata, purpureo-rufa, costis radiantibus circiter 18 subobsoletis, vel minime prominentibus instructa, concentrice, confertim striata; margines dorsales aequales, late concavi; linea cardinis aeque leviter concava; auriculae subaequales, antica valvæ dextrae inferne haud profunde sinuata; pagina interna rufescens.
Longit. 70 mm., alt. 70, diam. 16.
Habitat. – Franklin Island, 10 fathoms.
A large very thin species of which only a single right valve was obtained. It is remarkable for its rich purplish red or plum colour and the feebleness of the radiating costation. The costae are somewhat more pronounced towards the umbo than upon the rest of the surface, and are about twelve in number. Other intervening and more feeble ribs are developed at a later period of growth. The concentric sculpture consists of very fine thread-like lamellae which are more remote in the early stages of growth than at a later period. The auricles are striated with fine lines of growth, and anterior, in the valve described, exhibits a faint raised ray down the middle.
There does not seem to be any species bearing sufficient resemblance as to suggest a comparison with the present form. The Arctic P. greenlandicus of Sowerby has even a thinner shell, but in size, colour, and sculpture is quite distinct.”
(Smith, 1902: 212)