Page 5 - The Freshwater Mussels of Oklahoma

The external shell surface may also exhibit considerable variation. Many species
are rather smooth but others show various sculpture consisting of pustules, knobs, ridges,
fluting, wrinkles, folds, etc. The size, location and arrangement of these are often useful
in identification. Special sculpture may also be present on the embryonic or early
juvenile umbos and is called beak sculpture. This is also a useful identifying character
if present, but is usually eroded away while the individual is fairly young. Another
surface character that may be present is concentric rings of color or texture irregularities
that may represent changes in growth rate. In some areas these are useful in aging a
specimen but must be used carefully because other environmental or physiological
disturbances may cause the formation of false annual rings.
The external shell surface may have one or more angular or rounded ridges that
often radiate from the umbonal region to some part of the margin of the shell. If the ridge
extends posteriorly, it is called a posterior ridge and the area between the ridge and the
posterior margin is the posterior slope. If other ridges are present, there is often a
depressed area between the ridges called a sulcus.
The two shells or valves are connected by a hinge ligament. This tough, flexible
structure and the presence of the embryonic growth area, usually characterized by a bump
or raised area called the umbo, define the dorsal margin of the valves. On the interior
surface of the shell below the ligament and posterior to the umbo area are usually one or
two elongate, blade-like ridges called lateral teeth. Posterior to the lateral teeth are
usually one or more short peg-like projections called pseudocardinal teeth. All of these
teeth apparently help to keep the two valves in proper alignment by preventing any
slippage either in a dorsal/ventral or anterior/posterior plane. There are some species of
unionids in which the teeth are reduced or even absent. In others, the size, position,
number and form of the teeth vary widely and are often useful characters in identifying
shells.
Other internal shell characters include scars where muscles are attached. At the
anterior end of the shell (the end with the pseudocardinal teeth) are several of these scars.
The largest is the anterior adductor scar. Adjacent to it are usually two smaller scars,
the anterior retractor scar and the anterior protractor scar. At the posterior end of
the shell usually just below the end of the lateral teeth are two scars, a large posterior
adductor scar and a smaller posterior retractor scar. A series of small scars may be
present in the umbonal cavity area. These are the dorsal muscle scars. An impressed
line is usually visible extending from the region of the posterior adductor scar to the
region of the anterior adductor scar and running parallel to the ventral margin (the margin
opposite the ligament). This impressed line is the pallial line and is where the mantle is
attached to the shell. In some shells, there is a flattened area present between the lateral
and pseudocardinal teeth. This is called a hinge plate or interdentum. It may be absent
in some species. In some shells there may be one or two projections of shell on the
dorsal side. If the projection is posterior to the umbo it is a posterior wing. It is an
anterior wing if it occurs anterior to the umbo.
This discussion of the shell is a summary of the skeletal system of the mussel.
Obviously it is an exoskeleton and serves several functions. First of all it protects the soft
structures of the animal from mechanical injury due to predators, water currents,
abrasion, etc. Second, it provides support for the soft tissues. Third, its weight and

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