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Show 392 MOLLUSCA. ligament placed behind the hinge opens the valves by its contraction. A considerable number of bivalves are provided with what is termed a byssus, or a fasciculus of threads more or less loosely connected, which issues from the base of the foot, and by which the animal adheres to various bodies. It uses its foot to direct the threads and to agglutinate their extremities; it even reproduces them when cut, but the nature of the production is not thoroughly ascertained. Reaumur considered these threads as a secretion, spun and drawn from the sulcus of the foot; Poli thinks they are mere prolongations of tendinous fibres. The shell essentially consists of two pieces, called valves, to which in certain genera are added others, connected by a hinge that is sometimes simple and sometimes composed of a greater or smaller number of teeth and plates, which are received into corresponding cavities. There is usually a projecting part near the hinge called the summit or nates. Most of these shells fit closely when the animal approximates them, but there are several which exhibit gaping portions either before or at the extremities. FAMILY I. OSTRACEA. The mantle is open, without tubes or any particular aperture. The foot is either wanting in these Mollusca or is small; they are mostly fixed by the shell or byssus to rocks and other submerged bodies. Those which are free, seldom move except by acting on the water by suddenly closing their valves. In ~he first subdivision there is nothing but a muscular mass ~eachmg from one valve to the other, as seen by the single Impression left upon the shell. It is thought proper to class with them certain fossil shells, ACEPUALA' TESTACEA. 393 the valves of which do not even appeat· to have been held together by a ligament, but which covered each other like a vase and its cover, and were connected by muscles only. They form the genus AcAUDA, Brug.-OsTRACIT A, La Peyr., Of which M. de Lamarck makes a family that he names RuDISTA. The shells at·e thick, and of a solid or porous tissue. They are now divided into the RADIOL!TEs, Lam., In which the valves are striated from the centre to the circumference. The one is flat, the other thick, nearly conical and fixed( 1 ). SPHJE.RULITEs, Lameth., Where the valves are roughened by irregularly raised plates. It is also thought we may add the CALOEOLA, One valve of which is conical but free, and the other flat and even somewhat concave, so that they remind us of a shoe; and even the HIPPURITES, Where one valve is conical or cylindrical with two obtuse, longitudinal ridges on the inside; the base even appears to be divided into several cells by transverse septa(2); the other valve fits like a cover. The BATOLITIIEs, Montf. 334, Are cylindrical and straight Hippurites; they are frequently found (1) The species ofBrugiere, 173, f. 1, 23, which forms the genus AcAnnA, Lam., appears to be nothing more than a double epiphysis of the vertebra of some ceta· ceous animal. The DxscxNE, Lam., are Orbicula:; it is also thought that his Crani~ should be approximated to them. The J onAMIES of M. de France or BnosurT:t:s, Lam., are mere moulds of SramnuLITEs or at least of the bod!es always found in their interior, although they do not adapt themselves to their form. SeeM. Charles Desmoulins on the Splterulite:J. (2) See Desbayes, Ann. des Sc. Nat., June, 1825; and Ch. Desmoulins, loc. cit. Sever:U Hippurites have been described by La Peyrouse under the impl'Oper ~e of Ortlwceratitea. The Cornucopire of Thompson, Journ. de Phys. an. X, pl. 11• 18 also one of them. ' Vot. II.-2 Z |