_Lunule._ The lunated depressions, situated in the anterior and posterior slopes. In different species of Venus they are prominent characteristic marks, often of much service in ascertaining a species. Plate I. fig. 4, _b b_. _Ligament perforation._ The circular aperture, or perforation through which the ligament passes; by which the animal of the Anomia attaches itself to stones and other marine extraneous bodies, it is in general situated in the flat valve, though there are a few exceptions to the contrary. Plate I. fig. 7, _m_. _Hinge_ is the point at which bivalve shells are united; it is formed by the teeth of one valve inserting themselves between those of the other, or by the teeth of one valve fitting into the cavities or sockets of the opposite one. It is on the peculiar construction of the _hinge_ that the generic character of bivalve shells is principally founded, together with the general contour of the shell. Plate II. fig. 1 and 3, _q q q_. _Teeth of the hinge._ Upon the number and relative situation of the teeth principally depend the specific distinctions; they are of various forms, and very differently placed; some are single and large, others numerous and small, orbicular, spatuliform, laminated, &c. Some hinges have no visible teeth, and are termed _inarticulate_.—When a primary tooth has a groove or hollow in its centre, it is called complicated. Plate IV. Fig. 6, _e_; those with few teeth are termed _articulate_. Plate IV. fig. 1 and 12; and those with many teeth _multiarticulate_. Plate IV. fig. 2, 3, 4, &c. Plate I. fig. 6, and 8, _g g g g_. _Primary teeth_ are those teeth in general situated in the centre of the hinge, and are for the most part broad, large, and distinct, often elevated; and in general are inserted in a cavity in the opposite valve. They however differ very much in some shells, but may easily be distinguished. Plate IV. fig. 1, 3, and 7, _a a a a a_. These are also termed the Cardinal Teeth. _Lateral Teeth_, Plate IV. fig. 10, are teeth which diverge from the Umbo, and are in general long and flat, often double and divided by a groove or hollow. Plate IV. fig. 2, 4, 6, and 7. _b b b b_. _Double Teeth_, Plate IV. fig. 1, _c c_. _Incurved Teeth_ are those which are bent round, as in the single tooth of the Solen. Plate IV. fig. 8, _f f_. _Recurved Teeth_, are those which are bent backwards, as in the hinge of the Panopea and Spondylus. _Middle Teeth_, Plate IV. fig. 4, _d d_. These are also termed Cardinal Teeth. _Numerous Teeth_, are those small upright teeth, set in rows, of which the hinges of all the species of the genus Arca are formed. Plate IV. fig. 10, _k k_. _Cavity of the Hinge._ The hollow depression in which the ligament of the Ostrea is situated, generally of a triangular form. Plate IV. fig. 11, _g_.