"The _ligament_ is a membrane that connects the valves, and keeps the hinge in its proper place: it is always situated near the beaks. The ligament is very perceptible in the cockle, in _Pecten_, or scallop, in _Tellina_, &c.
"The _beaks_ are the most pointed parts of the bivalve shell (Plate 3, _c_.); when the valves are closed, the line where they meet is called the _seam_. (Plate 3, _d_.)
"The _anterior slope_ is that part of the shell where the ligament is placed, and is also called the _area_. (Plate 3, _e_.) The _posterior slope_, or _areola_, is the other side of the beaks. (_f._)
"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_.
"_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.