The shells of the brachiopods are different from those of the lamellibranches (bivalvular) by their plane of symmetry, perpendicular to the two valves.

Size

Shells of recent species measure approximately between 1mm and 8cm.

We are very far away from the "giant" Carboniferous Productus giganteus (Martin), which exceeded 30cm in width.

Lambdarina manifoldensis (Brunton & Champion), a tiny shell of 3mm broad, discovered by C. Champion in the lower Carboniferous of Staffordshire,U.K. "... no wonder they were overlooked by other geologists!"

Shell structure

Some shells (Inarticulata)are made of calcium phosphate and chitin, others (Articulata)of proteins and calcite. The periostracum, an organic matter, does not fossilize. The structure of the shell of many articulated brachiopods shows two different kind of successive layers : the outer primary layer is cryptocrystalline, whereas the inner secondary layer looks fibrous.

White, yellow, orange, pink, brown, gray, transparent or made green by algae... recent shells are covered with a brown cuticule called periostracum.

Ornamentation

Reticulate, striated, costellate, punctuated, thorny... the livery of a brachiopod can take many aspects.

The complete shell is often covered with ribs and wavy concentric ridges. In several species, shell even remains smooth.

Growth lines may also be visible especially on adult shells.

Shell porosity

Three structural types of shells can be distinguished :

1) endoponctate(Spiriferida, Thecideida, Rhynchonellida, Terbratulida ...).The shell is uniformly crossed by channels (caecums) to finish in perforations below the periostracum.These channels are filled out with the tissue of the mantle. The exact role of the caeca is stillin doubt.

2) pseudoponctate(Orthida, Strophomenida..).The shell is only pseudo-perforated.

3) imponctate(all Cambrian species). It is the most primitive structural form. The shell does not have pores.

Bivalvia or Lamellibranchia : aquatic molluscs with 2 valves and the gills in shape of plates ( oysters, clams, cockles...).

Mantle : fleshy membrane which covers the viscera of molluscs and which secrets the shell.