Brachiopods are sessile, benthic, filter-feeding, marine invertebrates. They first appear in Cambrian rocks and were abundant in Earth's oceans throughout the Paleozoic. They were also common in Mesozoic oceans, but are scarce in modern oceanic biotas. Brachiopods have two shells, called valves, that are usually calcareous (made of calcite - CaCO3 - calcium carbonate). Each shell of a brachiopod is bilaterally symmetrical, unlike each shell of a bivalve (clam).

Pentamerids are usually large brachiopods that are moderately common in Silurian dolostones of America's Midwest. The calcite shell has usually dissolved away, leaving an internal mold.

The brachiopods shown above may be Kirkidium. The brachiopods at top and bottom are hollow and lined with crystals - they have been geodized. The upper brachiopod has nice, large, whitish-gray calcite crystals.

Classification: Animalia, Brachiopoda, Articulata (a.k.a. Rhynchonelliformea), Pentamerida, Pentameridae

Stratigraphy: unrecorded/undisclosed Silurian unit (but possibly from the Wabash Formation, Ludlovian Series, Upper Silurian)

Locality: unrecorded/undisclosed site at or near the town of Swayzee, east-central Indiana, USA (but possibly from the Pipe Creek Junior Quarry)