ARCA. Auct. (Anglicè, a boat.) _Fam._ Arcacea, Lam.--_Descr._ Obliquely transverse, subquadrate, equivalve, or nearly so, inequilateral, thick, ventricose, longitudinally ribbed, dentated near the inner margins; hinge rectilinear, forming a flat, external area, upon which the ligament is spread in cross rows, and having a series of small, regular teeth, extending on both sides of the umbones in each valve; muscular impressions distant.--_Obs._ The shells composing this genus are easily distinguished from those of all other bivalve shells, by the straight, linear row of small, notched teeth, and by the area between the umbones. The genus _Cucullæa_ makes the nearest approach to it in this respect, but it may easily be known from it by the outermost teeth on each side of the row being oblique, and lengthened out; and also by the prominent edge of the muscular impression. These shells are found recent, in various marine localities; fossil, in the tertiary deposits. The Arca Noæ, formerly regarded as the type of this genus, has, with several other species, been separated from it under the name of Bysso-arca, by Swainson, on account of an hiatus in the ventral margin, to admit the passage of a byssus; this is not found in the true Arcæ. The true Arcæ are mostly tropical. Arca Antiquata, fig. 131. Bysso-arca Noæ, 132. ARCACEA. Lam. A family of the order Conchifera Dimyaria, characterized by a series of teeth placed on the hinge in a line. The genera may be distinguished as follows, 1. ARCA. Hinge straight; valves close. Fig. 131. 2. BYSSO-ARCA. Valves gaping. Fig. 132. 3. CUCULLÆA. Distant teeth oblique; posterior muscular impression prominent. Fig. 133. 4. PECTUNCULUS. Hinge curved. Fig. 134. 5. NUCULA. The same, with a pit in the centre of the hinge, including Myopara and Crenella. Figs. 135 to 137. 6. SOLENELLA. Fresh water, oval; a series of teeth on one side of the hinge, only two or three on the other. Fig. 138. ARCHAIAS. Montf. A genus of microscopic Foraminifera. ARCHONTE. Montf. HYALÆA, Auct. ARCINELLA. Schum. CHAMA Arcinella, Auct. ARCTICA. Schum. CYPRINA Icelandica, Auct. ARCUATED. (Arcus, an arch.) Bent in the form of an arch, as Dentalium, fig. 2. AREA. A flat space or disc, on any part of a shell. As for instance, the triangular space on the hinge of Arca, fig. 132, and Spondylus. ARENACEOUS. (Arena, sand.) Of a sandy texture, as the sand tubes surrounding the bodies of some of the Annellides, named Arenaria on this account. But the word is more commonly used to intimate the habits of the animal, burrowing with its shell in the sand. ARETHUSA. Montf. A genus of microscopic Foraminifera.