Opercula of Spiral Univalves.
Fig. 64, multispiral; 65, paucispiral; 66, concentric; 67, articulated; 68, radiated; 69, lammellated; 70, unguiculated.
BIVALVE SHELLS.
Bivalve shells, named Conchacea by Lamarck, are those which consist of two principal portions united to and folded upon each other by means of a hinge. The pieces united compose the shell, while each piece separately is called a valve. Considering the bivalve shell as a whole, it will be necessary, in the first instance, to describe the position in which it is to be observed, in order to give the student a clearly defined notion of what is intended, when terms expressive of height, depth, length, breadth, &c. are used, as well as when the anterior and posterior extremities are spoken of. For this purpose, we must suppose the animal to be living and creeping along the bed of the sea by means of its foot; where this foot protrudes, will be the ventral margin, and the opposite part the dorsal margin of the shell. There will then be a valve on each side; and if we further suppose the animal to be walking forward with its back to the observer, the right and left valves will correspond with his right and left sides.
Measurements.
The length will be measured from anterior (a) to posterior (p), and the lines of growth running in the same direction will consequently be longitudinal or concentric; transverse of some authors. The height will be from the umbones (u), to the ventral margin, and lines or bands in that direction are termed radiating; longitudinal, according to some authors.
The points from which the growth of the shell commences, are called the umbones; these usually turn towards the anterior part of the shell: if this circumstance fails to point out the anterior, it may in many cases be distinguished by the muscular impressions of the mantle. If this has a sinus or winding, it is always near the posterior muscular impression; and in all cases where there is an external ligament, it is on the posterior side.
There is sometimes an impression near the front of the umbones, which forms a semicircle on each valve; the space within this semicircle is called the lunule (wood-cut, fig. 71 and 72, l. l. l.); a corresponding depression, when it exists on the posterior margin near the umbones, is named the escutcheon.