Pen Shell (Subfossil Fragment)
Atrina cf. rigida (Stiff Pen Shell) or Pinna cf. carnea • Subfossil, Bivalve Invertebrate
Geological Period
Quaternary (Holocene Epoch)
Estimated Age
0-0.01 million years (Holocene)
Preservation Type
Unaltered shell material (Subfossil). The aragonite and calcite layers are largely preserved without significant mineralization, retaining original luster and translucency.
Condition Assessment
Fair to Good; it is a partial fragment rather than a complete valve. The edges show signs of breakage and wear from water transport.
Taxonomic Classification
Phylum: Mollusca, Class: Bivalvia, Order: Ostreida, Family: Pinnidae, Genus: Pinna or Atrina
Morphological Description
A triangular, wedge-shaped shell fragment featuring a distinctive pearly nacreous interior. The exterior displays concentric growth lines and a semi-translucent, thin-walled structure characteristic of the Pinnidae family. One edge appears straight (the hinge line area) while the body flares outward.
Rock Matrix
None present (loose specimen). Usually associated with marine sands, silts, or seagrass bed sediments.
Formation & Location
Common in coastal Quaternary deposits along the Atlantic coast, Gulf of Mexico, and Indo-Pacific regions. Often found in recent beach wash or shallow marine sediments.
Size & Dimensions
Approximately 5-8 cm in length; consistent with a partial fragment of a larger specimen that can reach 20-30 cm.
Value & Rarity
Estimated Market Value
$2.00 - $10.00 USD
Auction Estimate
$5.00 - $15.00 USD (typically sold in bulk lots)
Rarity Assessment
Common; Pen shells are prolific in modern and recent geological environments.
Scientific Significance
Provides information on recent marine biodiversity and local paleoenvironmental conditions in shallow-water shelf ecosystems. As a subfossil, it tracks Holocene sea-level changes and species distribution.
Preparation Recommendations
Requires minimal intervention. Clean with soft brush and distilled water to remove salt; store in a stable, low-humidity environment to prevent delamination of nacreous layers.
Notable Features
Retains high degree of original nacre (mother-of-pearl) luster; shows classic thin-walled structure that distinguishes Pinnids from other large bivalves.