Seen here is a brachiopod, a group of 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).

Classification: Animalia, Brachiopoda, Articulata (a.k.a. Rhynchonelliformea), Terebratulida, Terebrataliidae

Locality: 65 feet depth, offshore from La Jolla, north of San Diego, far-southern California, USA

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See info. at: en.wikipedia.org/key/Brachiopod and

en.wikipedia.org/key/Terebratulida