_Ligament_ of the Hinge, or cartilage, is that flexible fibrous substance by which the valves are united, and the hinges kept in their proper places; generally situated under the beaks of the shell. Plate II. fig. 1, 3, and 4, _l l l_; Plate IV. fig. 3, _n_. _Beak_ is the extreme point of the summit of bivalves, which in many species turns spirally downwards or to the one side, as in some species of Venus, &c. From this circumstance it is seldom the highest part of the shell. Plate II. fig. 5, _r r_. _Seam._ When the valves are closed, the line of separation between them is so called. Plate II. fig. 6, _t t_. _Umbo._ That part situated immediately under the _beak_. Plate I. fig. 6, _w_; and Plate II. fig. 4, _w_. _Ears._ The processes on each side of the beak, in most species of that division of Ostrea, called Pecten; some have one ear very large, and the other small; and some are scarcely observable on one side. Pl. II. fig. 2, _h h_. _Superior Ear._ Plate IV. fig. 11, _h_. _Inferior Ear._ Do. do. _i_. _Margin._ The extreme edge of the whole shell, or the circumference of either valve, all round. Plate I. fig. 4, _p_; and Plate II. fig. 1, _p_. _Crenulated margin._ That fine notched edge of shells, which unite into notches in the opposite valve, as genus Donax. Plate IV. fig. 6, _m m_. _Striæ_ are fine thread-like lines, generally on the exterior surface of shells, and are sometimes both longitudinal and transverse. When the striæ of shells appear indistinct, as if worn out, it is termed _obsolete striæ_. In some instances the insides of shells are striated; for example, the Fasciolaria Tulipa. The character of the striæ is often of much use in distinguishing species. Plate II. fig 2, _f_. _Right Valve_ is that valve which, when viewed with the inside uppermost, has the anterior slope pointing to the right hand. Plate I. fig. 8; and Plate IV. figs. 5 and 6, _B_. _Left Valve._ The opposite of the above; the anterior slope points to the left hand, when viewed from the inside. Plate IV. figs. 3, 6, 7, 9, &c. _Length of the Shell_ is taken from the ligament, or the beak, to the opposite margin. For example, Mytilus: it is longer than it is broad, and the Solens are broader than long. Plate I. fig. 5, _u u_; and Plate II. fig. 4, _v v_. _Breadth_ is measured from the most extreme edge of the anterior and posterior slopes, being in a contrary direction from its length. Many shells are _broader than long_, such as most of the Myæ, Solenæ, Tellinæ, &c.; and the Mytilus, Ostrea, Pinna, &c., are in general longer than broad. Plate I. fig. 5, _v v_. Plate II. fig. 4, _v v_. _Byssus_, or _beard_, is an appendage composed of filaments of a silky texture, by which some of the Bivalves fasten themselves to their beds, such as the Mytilus, Pinnæ, &c. Plate II. fig. 6, _s_. UNIVALVE.