"The _lunula_ is a crescent-like depression on either the area or areola. The edge of the valve is called the _margin_; it is often finely _crenulated_, or toothed. The interior of the valve is called the _cavity_. (_g._) In the valves of this ark-shell here are two broad marks, shining and glossy. (_g._) In those of the oyster and muscle that I now show you, there is but one. These marks are _muscular_ impressions; they are the parts where the muscles of the animal have been affixed, and are termed _cicatrix_. [Illustration: Plate 3.] [Sidenote: TERMS--UNIVALVES.] "_Ears_ are two processes on each side of the beak; the _Pecten_, or scallop, is an example. (Plate 3, _i_.) "_Sinus_, in _bivalve_ shells, is a small hollow in the hinge. "_Byssus_, or beard, is an appendage composed of silky threads, by which the muscle and _Pinna_ fasten themselves to the rocks. (Plate 3, _f_.) "_Cordiform_ is a term applied to heart-shaped shells. "A _cartilage_ is the same as a ligament. When the valves of a shell are very nearly flat, they are said to be _compressed_: when a valve has teeth, it is said to be _dentated_. "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. [Sidenote: 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.) "The _beak_ is the lengthened process (_g_) at the lower part of the shell. "The _canal_, or _gutter_, runs through the beak. (Both these parts are perceptible in _Murex_ and _Strombus_, Plate 7.) "_Sutures_ are spiral lines which separate the whorls; they are sometimes crenated, or notched, sometimes sulcated, or furrowed.