"When the valves of a shell do not shut close, they are said to be gaping. ([Plate 4], Mya.)
"A muscle is a fleshy, pliant organ, by which the animal is attached to its shell. I have already pointed out to you the impressions of those muscles within bivalve shells.
"A suture is a toothed joint, in bivalves.
"A shell with ears is said to be auricled.
"The third division, Univalve shells, have also their several parts. The first section has a regular spire. Here are two shells of this section, Voluta and Buccinum, both sawed asunder, in order to show the interior structure of the shell. ([Plate 3.]) The aperture, or opening, being turned towards you, the front of a univalve is seen; reverse it, and you see the back.
TERMS—
UNIVALVES.
"The top, or highest part, is the apex; (a) the lowest part is the base of the shell. (b.)
"The spire (c) is formed of wreaths, or whorls, (ddd) which terminate in the apex: the lowest whorl is the body of the shell. (e.)
"The aperture, opening or mouth, (f) as it is sometimes called, is on the right-hand when the front of the shell is turned towards you. The aperture is an important distinction in univalves: some genera have a circular opening, as the Turbo, or periwincle; some longitudinal, as the cowry; others semi-lunar, as the Helix, or snail genus. (For examples of these apertures see Plates 1 and 7.)