Cycles. This genus comprehends some shells which Linnaeus had associated with Tellens ; but which Brugulere and La marck have separated from them, from their wanting the fold on the anterior side. The characters are : A suborbicu lar, or, in a small degree, transverse shell, without fold on the anterior side ; the ligament external and bulging ; two or three cardinal teeth, and elongated, la melliform, inserted lateral teeth. They are fresh water shells.
Venus. This genus of Linnaeus is form. ed by shells which are regular, subor bicular bivalves, provided with a crescent and a corslet ; three cardinal teeth near together, and sometimes one or two la. teral teeth. The valves of this genus are generally pretty gibbous and thick ; con. stantly equal, and nearly of a triangular form Lamarck divides this genus into two. 1. Venus, a suborbicular or trans. verse shell, with three approximated car dinal teeth, the lateral ones being more or less diverging. 2. Meretrix, a trans verse or orbicular shell, with three ap proximated cardinal teeth, and one isolat ed tooth under the crescent. The genus Caspa, mentioned above, is proposed by Brugulere and Lamarck to comprise several of the Venus's of Linnaeus.
Ungulina. A regularly formed long bivalve ; the hinge formed by a very small tooth between two oblique dents : the valves bearing the form of the finger nail. There is but one species in this genus. The hinge bears some resem blance to that of eardium, but the general form of the shell differs much from that of Cardin.
Donax. A regular, transverse, inequi lateral bivalve, with three cardinal teeth, and one or two separated lateral teeth. Shells of this genus have much of the triangular form, their sides very unequal ; they are solid, thick, flattened at the in ferior, and rounded at the opposite, ex tremity. The ligament, which, in most shells with equal valves, is placed above the summit, is in these shells unequally distributed above and below it. Above, it is narrow and short ; and below, it is thick, nearly round, and fills the cavity of the corslet exactly.
Mactra. A bivalve shell, regular, trans verse, inequilateral, and a little gaping, the cardinal tooth having a pit for the ligament ; lateral teeth, compressed and inserted, or none.
Lamarck divides this genus into three, 1.Mactra, a transverse, inequilateral shell,
a little the cardinal tooth folded in a furrow, articulating with that of the opposite valve, and accompanying a pit for the ligament ; two lateral and insert ed teeth, such as Mactra stultorum. 2. Lutraria, a transverse, inequilateral shell,. gaping at the extremities; two cardinal teeth, oblique, and diverging, accompa nying a large pit for the ligament ; no lateral teeth such is Mactra lutrarin, Linnaeus. 3. Paphia, a subtransverse, inequilateral shell, the valves close : a pit for the ligament on the beak, between or near to the teeth of the hinge, as in Venus divaricata, Linnaeus.
Crassateila. A genus formed by La marck of shells unknown to Linnaeus, and which Brugulere had placed in the genus Mactra. The generic characters are, an inequilateral, subtransverse shell, .with close valves, with a sunk crescent or corslet ; the ligament internal ; the pit for the ligament placed under the beaks, above the teeth of the hinge. There are two fossil, and one recent species under this genus. These shells are remarkable for the thickness of the valves, and for their two deep muscular impressions.
Trigonia. This is a genus formed by Lamarck, the species of which are known only in a fossil shape. The characters of the genus are, an inequilateral subtri gonal shell, the hinge formed by two large, flat, diverging teeth, transversely grooved. These are the Cunei of Da Costa.
Hiatella. A genus formed by Daudin on two shells from Tranquebar, which ap pear to be intermediate between the ge nera Trigonia and Tridacna ; the charac ters are, a transverse, irregular bivalve shell, gaping in its superior edge ; its hinge a tooth in each valve, inserted in a groove in the opposite valve.
Cuctifiza. A gibbous, subtransverse inequilateral shell. with detached beaks the hinge in a straight line, having a range of numerous teeth, transverse inserted ; and at the extremities two or three pa rallel rib-like teeth,. the ligaments exter nal. This genus is founded by Lamarck on a recent shell from the Indian Sea, and on a fossil shell from Beauvais, which Brugulere had placed among the Arks, from which these shells duller in the ribs placed at the extremities of the hinge.