Half-naked Pen Shell

Shell size to 240 mm; shell fan-shaped, triangular. Hinge area straight, representing larger side of triangle. Surface sculpture of narrow ribs separated by larger interspaces; ribs bearing regularly spaced, fluted spines. Ribs present on about top half of fully grown valve. Large muscle scar well within (below) border of nacreous area (pallial line; see photo [middle] of open pen shell by Amy Tripp showing position of muscles). Byssus at pointed extremity anchors penshell into seagrass bottom. Gaping, narrower side of triangle oriented upward. Color dark-olive brown. This picture of a live specimen at low tide was taken by Amy Tripp near Marco Island. The mantle is rigid orange-red structures are the ctenidia, or gills. Compare with Atrina rigida (Lightfoot, 1786), which has ribs extending to bottom half of fully grown shells, a muscle scar jutting above the nacreous area, and grayish- to greenish-brown mantle.