BIVALVE SHELLS.
Calceola. A regular bivalve shell, with unequal valves, the largest being in the form of a half sandal, and the smallest flat, semiorbicular, and resembling -an operculuna ; the hinge with from one to three small teeth This shell is only known as a fossil, and is considered by Gmelin as an Anomia (Anomia sandali um). It is very thick, and about an inch in length. Its back is flattened; its interi or is striated longitudinally, and its valve, or operculum, concentrically.
Terebratula, a regular bivalve shell, with unequal valves, fixing itself by a liga ment or short tube ; the largest valve per forated at its summit, which is prominent and recurved ; hinge with two teeth. The regularity of the valves, the absence of any operculum, the position of the hole, and the form of the hinge, separate this genus from that of Anothia.
Crania, a regular bivalve shell, with un equal valves, the inferior almost smooth, and nearly round, and pierced in its inter nal face with three unequal and oblique holes ; the superior valve very convex, furnished internally with two projecting callosities The three holes in the inferi or valve of this shell induced Lamarck and Brugulere to separate it from the Ane mia, and place it in a distinct genus. The species described by Linnaus is named by him Anomia cramolaris.
Placuna, an irregular bivalve flat shell ; the interior hinge composed of two di vering ridges, which serve for the at tachment of a ligament. Linnaus has ar ranged two species, Anomia placenta, and Anomia sells, under the genus Ane mia.
Hyalza, a regular bivalve shell, with unequal, bulging connies, transparent valves, gaping under the beak, and tri cuspidated at the base. This genus is formed by Lamarck, from a shell describ ed by Forskal, and denominated in Gme lin's Linnaus Anomia tridentate, canasta, and retusa. The Clyo pyramidata of Lin nails are also referable to this genus. Lingula, a long flat shell, formed of two valves, nearly equal, truncated in the forepart ; the hinge without teeth ; the beak of the valves pointed, and joined to a tendinous tube, which is a ligament to the shell, and fixes it to other solid bo dies. Linnzus, who knew only of one
valve of this shell, termed it Patilla un guis.
Corbula is an unequal-valved, sub transverse, smooth, regular shell ; with a conical cardinal tooth, curved or turned upwards, 'on each valve ; interior liga ment, and two muscular impressions. This genus is characterized by the ine quality of its valves, one of them being nearly • third larger than the other. These shells are found fossil at Grignon, near to Versailles.
Orbicula is a genus formed by Lamarck from a shell which was discovered by Muller, and which was named by him Pa tella anomala. Its differing so entirely from Patella in being a bivalve, and its being inhabited by an animal totally dif ferent from that of the Patella, has induc ed Lamarck thus to separate it from the genus Patella. This shell is orbicular, but rather flattened, and composed of two valves ; by the lowest of which, which is extremely delicate, it adheres to other bodies. Its hinge is not known.
Pandora is a regular inequivalved and inequillteral shell; with two cardinal, ob. long, unequal, and diverging teeth, in the upper valve ; and two oblong pits in the other valve ; and interior ligament, and two muscular impressions. The shell which gives rise to this genus is Tellina iniequivalvis, of Linnzus ; the inequality of the valves being the circumstance which is supposed to warrant the sepa ration. This shell is named Pandora mar garitacea, by Brugutere.
Lima. An inequilateral•eared bivalve; the valves a little gaping OS one side ; hinge without teeth ; ligament external ; beaks separated. This genus is founded chiefly on the Ostrea hunt of Linnzus, now named by Brugulere, Lima squamos. The separation of these shells from the Pectens, on the idea of their forming a byssus, whilst the Pectens do not, re quires, it should seem, a more accurate knowledge of the habits of the animals which occupy the different Pectens.