download pdf

82 Aifierican Seashells
other and to determine which is the anterior or posterior end, for many
identification features are used in relation to these orientations. The dorsal
or upper inargin is located on the beak or hinge side; the ventral margin is
the opposite side. The beaks usually are pointed or curved toward the ante-
rior end which is generally the less pointed end of the shell. The ligament
in the great majority of cases is posterior to the beaks. When present, the
heart-shaped impression called the lunule is anterior to the beaks. When a
clam is placed on its ventral margins on the table with the dorsal hinge mar-
gin up, and with the anterior end away from the observer, the right valve is
on the right, the left valve to the left. Another quick way is to observe the
concave, interior of a valve with the hinge margin away from the observer
and to locate the U-shaped pallial sinus impression (see below). If the sinus
opens toward the left, it is a left valve, and vice versa for the right valve.
/^
I
NK
POSTERIOR
LPtTCRAU
Tooth
LIGAMENT
'ventral MftAClN
'Pallial line
Figure 27. Parts of the hivah-e shell.
In most bivalves, the two valves are of the same size (equivahe), but in
some genera one valve is larger and slightly overlaps the other (ineqjiivalve)
In Ostrea, Pandora and Lyonsia, the left valve is the larger; in Corbula, the
right valve is the larger. A bivalve is said to be equilateral when the beak is
midway between the anterior and posterior ends of the valve. Most bivalves,
however, are inequilateral with the beak placed nearer one end.
In many forms, the margins of the valves do not fit closely together, but
have an opening called the gape somewhere along the margin. In the Soft-
shelled Clam, My a, the gape is posterior and through it protrudes the siphon
(siphonal gape); in Rocellaria it is anterior and large and serves for the
.