History of British animals - University of Guam Marine Laboratory

408 MOLLUSCA. SIPHONIDA.SIPHONIDA.I. One syphon only, formed by the union of the cloak posteriorly.a. Furnished with a byssus; marine. Mytiiid^e.Mytilus.Modiolus.Lithodomus.aa. Destitute of a byssus; inhabiting fresh water. Unionid^e.b.Hinge simple.Anodon.(Iridina.)Car-b b.Hinge with teeth.Unio.Alismadon.A A. Shell with 'prominent beaks, the ligaments internal.ditada;.Cardita.Venericardia.Crassatetta.A. Shell transverse ; beaks obsolete, ligament, linear and external.II. Cloak closed anteally and retrally, with three openings towardsthe middle of the ventral margin, the first for thebyssus, the middle one for the entrance of water, and theanal posteriorone. (Tridacnad^e.)III. Cloak open anteally, with two retral syphons, the onebranchial and the other anal, which are sometimes producedinto united or separate tubes. A foot and two adductormuscles.A. Shell heart-shaped ; beaks prominent, incurved, withstrong hinge-teeth ; ligament external.a. Shell adhering, irregular, inequivalve ; anterior apertureof the cloak small, corresponding with the size of thefoot. CHAMADiE.Chama.(Diceras)(Etheria.)a a. Shell free, regular, equivalve ; foot large. Cabdiad^:.IsocardiaHippopodhim.Cardium.Pholadomya(Cypricardia.)