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84 Ai?]erican Seash ells
possesses a number of useful identification features. The large muscles which
serve to close the valves leave round impressions on the surface. When two
muscles are present, as in the venus, lucine, tellin and other clams, they are
known as the anterior and posterior muscle scars respectively. The fine,
single-lined impression produced by the muscular edge of the mantle is
known as the pallial line. The pallial line may have a U-shaped notch at
the posterior end of the valve indicating the presence of a siphon and its
siphonal muscles. This is known as the pallial sinus. It is entirely absent in
genera possessing no retractile siphons.
The hinge. This is one of the most important identification features
in the bivalves, and often many hours of fruitless search can be avoided
when the major types of hinges and their various parts are understood.
There are many types of hinges from those without teeth {edentulous) to
those with a complex pattern. We have figured below some of the major
types of hinges. The teeth are distinguished as cardinals, or those imme-
diately below the umbo, and the laterals, or those on either side of the cardi-
nals. In many inequilateral bivalves the teeth have become so distorted or
set out of place that it is often difficult to distinguish the cardinals from the
laterals or to determine which ones are absent. We have labeled the teeth
in several groups in the systematic section of this book to overcome this
difficulty. In Chanm, for instance, the cardinals have been pushed up into
the umbo and have become a mere ridge, while the strong anterior lateral
has become nearly central and simulates a cardinal.
Sculpture. In many groups, such as the scallops (Pecten), sculpture is
of paramount importance in determining species. In most other groups it is
used in conjunction with other characters. There are two major types of
sculpture concentric and radial—and both of these may be present in many
forms, such as ridges, ribs, nodules, spines, foliaceous processes (leaf-like),
threads, beads, indented striae (fine lines), etc. Concentric growth lines of
varying degree of development are seen on most bivalves. They are always
parallel to the margins of the valves, may be exceedingly fine or very coarse,
and they generally indicate former growth and resting stages. Radial sculp-
ture, running from the umbones to the lower or end margins of the valves,
is exemplified in the ribs of Cardium (pi. 32), Pectejj (pi. 33) and others.
Concentric and radial sculpture may occur together to form a cancellate
sculpture as in Chione cancellata (pi. 39h). In a few genera, such as Foromya,
the valve's surface may be granulose, as if finely sugar-coated.
The periostracum or protective chitinous sheath overlaying the exterior
of the valves is present in most bivalves. It may be extremely thin and trans-
parent so that it imparts a high gloss to the shell, or it may be thick and
matted or even very coarse and stringy so that the valves appear to be
bearded, as in Volsella and Area.