Hello Thomas,

In response to your question, I submit the text of a short article I
wrote, which appeared in the June 2000 issue of American Conchologist:

WHEN IS A BIVALVE NOT A BIVALVE?

     When it’s a Brachiopod!  Brachiopods, commonly known as "lamp
shells", are a phylum of marine invertebrates possessing a calcareous
bivalved shell, superficially resembling those of bivalved molluscs.  In
fact, these strange animals were not recognized as distinct from
molluscs until the middle of the nineteenth century.  The shell and the
thin outer periostracum of a Brachiopod are secreted by the mantle, just
as in molluscs.  However, should you open the shell of a living
specimen, you would realize at once that the creature resting in your
hand had little in common with a clam.  Most of the shell’s interior is
occupied by the lophophore, a two or three-lobed fleshy structure
covered with minute, sticky tentacles which capture microscopic plankton
from the ocean water and pass it to the central mouth.  The shell valves
of Brachiopods are oriented dorsally and ventrally (upper and lower),
rather than bilaterally (left and right) as in molluscs.  Some molluscan
groups such as Pecten (scallops), Spondylus (thorny oysters), and Ostrea
(oysters) give the impression of an upper and lower shell; but their
internal anatomy reveals that the valves are actually arranged
bilaterally, and the animal lives its life lying on one of its sides
(the left side in Ostrea, the right side in Spondylus and Pecten).  The
ventral valve of most lamp shells is larger and more convex than the
dorsal, and is usually perforated near the umbo by a hole, or foramen,
through which a cord-like stalk, the pedicel or peduncle, once passed,
anchoring the animal to the substrate.
     Lamp shells are one of the most ancient groups of animals.  In
fact, the genus Lingula is the oldest animal genus still represented by
living species, having remained relatively unchanged since the Cambrian
Period, some 600,000,000 years ago.  Brachiopods appear to be on a long
downhill slope toward extinction.  Already abundant during the early
Cambrian, they reached their peak of development in the Ordovician
Period, 450,000,000 years ago, and have gradually become less common in
the fossil record since then.  Over 30,000 fossil forms have been
described, but only about 250 species remain alive today, sporadically
distributed around the world, as last remnants of once widespread animal
groups tend to be.  They inhabit both shallow and moderately deep water,
but are seldom found off the continental shelf.  Fossil forms 30 cm in
diameter are known, but most are far smaller.  The largest living
species are about a third that size, and the majority of extant forms do
not exceed 30 mm or so in width.  Brachiopod shells may be smooth or
ribbed, usually with concentric growth lines, and are occasionally
sculptured with flutes or spines.  Shell color typically ranges from
grey through dull yellowish, but a few species are bright red or orange.
     Two principle classes are recognized in the phylum - the
Articulata, in which the shell valves are connected by a toothed hinge,
much as in bivalve molluscs; and the Inarticulata, whose valves are held
together only by the muscles and other tissues of the animal.
Inarticulate Brachiopods (Lingula, Glottidia, etc.) typically live in
vertical burrows in the sand or mud.  Their pedicel is long and thick,
and contains muscle fibers, the contraction of which pulls the animal
deeper into its burrow.  The shell valves are roughly equal in size,
relatively thin and often somewhat flexible, only moderately impregnated
with calcium phosphate crystals.
     Most specimens which reach the hands of collectors are of the
articulate type (Magellania, Terebratella, Terebratula, etc.).  The
hinge of such a shell has several simple teeth, plus interlocking
processes which prevent the shell from opening more than 10 to 15
degrees of gape.  These projections also prevent disarticulation of the
valves, except by breaking the hinge.  The shells of articulate
Brachiopods are mineralized with calcium carbonate, like those of
molluscs, but are softer than most mollusc shells, and do not stand up
well to bleach and other harsh cleaning agents.  The inner surface of
the dorsal valve is often specially sculptured to allow the lophophore
to fit snugly against it.  In many species, a delicate, calcified loop,
the brachidium, extends from the hinge area of the dorsal valve into the
shell interior, forming a support or internal "skeleton" for the large,
soft lophophore.  In some species, this structure is quite complex, with
varied twists, turns and branches, a wonder to behold.  Unfortunately it
is extremely fragile, and seldom survives the rigors of shell cleaning.
Efforts to remove the soft parts of the animal through the narrow gap of
the shell margin almost invariably result in shattering of the
lophophore support.  The chances of keeping the structure intact are
better if the hinge is broken and the shell opened wide, but even then
it can be a challenge to remove all the soft tissues without damaging
it.  The large ventral valve of articulate Brachiopods extends
posteriorly past the margin of the dorsal valve, and its apex is often
drawn out and reflected upward like the spout of a Roman oil lamp -
hence the name "lamp shells".  Because of this arangement, the rather
short pedicel emerges on the dorsal side of the shell.  Consequently, in
order to attach to the substrate, the animal must be oriented upside
down, with the ventral valve facing upward.  Articulate Brachiopods are
usually found on rocky or other hard surfaces.  In both Brachiopod
classes though, a few forms have lost the pedicel altogether, and cement
the lower valve directly to the substrate, much like an oyster.  Such
forms include the genera Crania (Inarticulata) and Lacazella (Articulata).
     Even though many lamp shell species are locally common and
frequently can be found in shallow water, many collectors are not
familiar with them, because few books on seashells include them, and few
shell dealers offer them. Still, a small assemblage of these unusual
shells can make an interesting addition to a collection, particularly
when the details of their natural history and ancient lineage are known.