Area. An incquilateral bivalve shell the hinge composed of numerous teeth, which lock in the interstices of those'of the opposite valve, and which range in a straight, angular, or curved line. The liga ment external. This genus of Linnxus has been subdivided by Lamarck into three genera. 1. Arca, with the teeth of the hinge in a straight lire, as the Arca note. 2. Pectunciihis, with the hinge in a curved line ; and S. Nucula, which de serves a distinct notice.
Nucula. This genus has been formed by Lamarck from Area nucleus, Linn. Daudin has also added two more species. The shells of this genus evidently differ from the Arks, in having their insides resplendent with the mother of pearl, and in having a large tooth at the hinge. The characters are, an inequilateral, al most triangular or oblong shell ; the hinge in a broken line, beset with nume rous transverse and parallel teeth ; one oblique cardinal tooth out of the line ; the beaks contiguous, and turned for wards.
Unio. A transverse shell, with three muscular impressions ; an irregular, cal lous cardinal tooth, stretching on one side under the corslet, and articulating with one of the opposite valve. In this
genus, formed by Brugutere, are placed the Ilya margaritifera and Pictorum of A n min n t a. A regular transverse bivalve shell, having three muscular impressions; a simple hinge without any tooth. In this genus are placed Mytilus cygneus, &maims, and fiaviatilis, of Linnzus, with three other species.
Mytilus. A regular transverse shell with equal valves, shutting close, a hinge without, or with one or two teeth. La marck divides the genus Mytilus into four genera. 1. Mytilus, a longitudinal shell, with pointed terminating beaks ; one muscular impression, and a hinge most frequently without teeth 2. Modiolus, a subtransverse shell, the posteriiir extre mity very short ; the beaks sunk upon the short aide of the shell; one muscular im pression ; the hinge simple, without teeth 3. Avicula. 4. Anodonta.
Pinna. The characters of this genus are, a regular bivalve shell, with equal valves, wedge formed, pointed at its base, gaping at its superior edge ; hinge with out teeth, and lateral ligament very long.