Pleuromya sp.

? Pleuromya sp.

Fig. 13.

1925 Lamellibranchiate 1; Hägg 1925: 48, pl. 5: 21, 21a.

Material. — One specimen (NRM-PZ Mo 149164), internal mold with fragments of the shell preserved, from the upper Paleocene of Fossildalen, Spitsbergen, Svalbard.

Description. —Shell elongate (L 11.7 mm), inequilateral with orthogyrate to opisthogyrate beaks displaced towards anterior. Anteroventral margin straight, anterior margin evenly rounded, ventral margin broadly rounded, passing smoothly into acutely rounded posterior margin. Posterodorsal margin straight, hinge without lateral teeth and apparently edentulous. Opisthodethic part of ligament visible. Posterior adductor muscle scar small, pallial sinus present, deep but not seen completely. External ornament of commarginal growth lines.

Remarks. —This species is most similar to Lyonsia sp. 1 from the Selandian (middle Paleocene) Sonja Lens in Nuussuaq, western Greenland (Petersen and Vedelsby 2000), with respect to the shape, the position of the beak, external ornament, inflation, and size. The available features visible on the material from Spitsbergen, especially a moderately inflated, equivalved shell, deep pallial sinus, and ornament composed of commarginal growth lines, suggest that this species does not belong to Lyonsia Turton, 1822. Lyonsia has a relatively shallow pallial sinus and ornament composed of commarginal lines superimposed on radial rows of spines (Prezant 1981; Coan et al. 2000; Coan and Valentich-Scott 2012). Instead, the specimen is more similar to Pleuromya Agassiz, 1842 (e.g., Duff 1978), which has a deep pallial sinus and ornament composed of commarginal growth lines. The interpretation presented herein should be treated as preliminary because only one specimen was available.