Chama. This genus is appointed, by Brugulere and Lamarck, to those shells only which, in particular, possess unequal valves, and are adherent. The charac ters of the genus-therefore are: a bivalve shell, with adhering unequal valves ; the hinge composed of one oblique thick tooth, which may be crenelated or rough, and is articulated in a cavity in the oppo site valve.
It was necessary, therefore, to esta blish other genera for the reception of those shells which thus became excluded from the genus Chima. Hence Bru gulere formed two new genera; Cardita, two teeth in the hinge : and Tri dacna, possessing the same number, hut having the edges of the crescent crenula ted and gaping. Lamarck has proceed ed still further; be has subdivided the genus Cardita from an isolated tooth, situ ated, in some species, under the corslet, as in Cardium isocardia, and makes this his genus Isocardia. He also makes an other genus Hippopus, the species of which are subtransverse, inequilateral shells ; the hinge with two compressed, inverted teeth, the crescent filled up: of this genus is Chama gigax. But it is ne cessary still further to particularize these new genera.
Cardita, a loose inequilateral shell, the hinge with two teeth, the one at the base of the left valve, and the other longitudi nal, parallel with the anterior face.
Hippopus. An inequilateral, subtrans verse shell; the hinge with two compress ed and inverted teeth ; the crescent filled up. Hippopus, it is to be remembered, differs from Tridacna, by having its cre nate tilled, whilst in Tridacna it is gaping.
Tridacna. An incquilateral subtrans verse shell ; hinge with two teeth, com pressed and inverted; the crescent gap-, ing. The Chama gigas of Linmeus (the Tridacna gigas of Lamarck) is the only species under this genus.
Cardium. This genus remains unalter ed by the French naturalists. The charac• ters being, a sub-cordiform shell, with valves dentated on their edges; the hinge with four teeth; two cardinal, approxi mating obliquely in each valve, and arti culating across with their correspondent teeth the lateral teeth distant, and in verted in the opposite valve.
Acardo. Is a genus very properly esta blished by Commerson. It is founded on
two species: the generic characters of which are, a shell composed of two, near ly equal, flatwalves ; having neither hinge nor ligament ; a muscular impression ap pearing on the centre of each valve.
Radiolites. This is a genus approxi mating to the preceding, but verypro perly separated from it by the form of the shell. The shell is of an irregular form, of unequal valves,striated on the outside ; the inferior valve turbinsted ; the supe rior convex or conical ; no hinge nor li gament.
Erodona. This genus was formed, by Hamlin, from two shells in the cabinet of Favannes. It is intermediate between the Mya and Mactra It is a subtransverse, bivalve shell, irregular and gaping; one of the valves having a hollow tooth t and the other a pit between two projections; the ligament inserted on the tooth and in the pit.
Mya, is a transverse shell, gaping at both ends : the ligament on the inside; the left valve having a cardinal tooth, compressed, rounded, perpendicular to the valve, and giving attachment to the ligament. This is the original genus of Linnaeus, from which Brugulere and La marck have withdrawn many of the spe cies, to place them under the genera Vulsella, Glycemeris, and Unio ; leaving only those sea-shells which have a very projecting cardinal tooth in one of their valves.
Glycemeris. A transverse shell, gap ing at both extremities; a protuberant hinge without teeth ; the ligament on the outer part. Such is Mys glycemeris of Linnaeus.
Solen. A transverse shell, gaping at both extremities: the upper edge straight, or nearly so ; two or three teeth at the hinge, furnished by both valves.
Tellina. Considerable confusion has arisen from the admission of species which by no means belong to this genus. Bru gulere and Lamarck have divided the Teller's of Linnaeus into two new genera. 1. Tellina, a transverse or orbicular shell, with equal valves, with a fold on the an terior side : one or two cardinal teeth, and two separated lateral teeth, as in Tellina virgata. 2. Caspa, a transverse shell, with two teeth in one valve, and one tooth in the opposite valve, shutting between those of the other valve, as in Tellina angulata.