_Unio_, taken chiefly from _Mya_. _Hyria_ _Mya_. _Anodon_ _Mytilus_. _Iridina_ Very rare genus. [Sidenote: UNIO. ANODON. DICERAS.] "_Unio_ has two teeth on each valve; one is cardinal, the other lengthened out. The ligament is exterior--the shell pearly. _Unio pictorum_ is common in rivers. The shell is used to hold small masses of gold or silver for artists, under the name of _shell-gold_. "_Anodon_ is also to be met with in our rivers. "_A. anatina_ is eaten by ducks and crows. The latter, when the shell proves too hard to penetrate, mount with it into the air, and letting it fall, pick out the fish from the broken shell. * * * * * "_Chamacea_ has only three genera. _Diceras._ _Chama_ _Chama_. _Etheria_, a rare genus, from the Indies and Madagascar. * * * * * "_Diceras_ is a fossil genus--only two species known according to Lamarck. [Sidenote: CHAMA.] "Linnæus had assembled in his genus, _Chama_, shells with equal and with unequal valves, shells fixed to other marine bodies, with those that are free; some with _one_, others with _two_ muscular impressions. In the present genus, _Chama_, the shells are irregular, thick, scaly, or spinous. The hinge has one thick tooth, often notched: the beaks are bent inwards. They are found in the Indian, American, and Mediterranean seas. There are several fossil species. "The first order, _Bimusculosa_, is finished. In our next lesson we shall proceed to the families and genera contained in the second, _Unimusculosa_." CHAPTER VIII. [Sidenote: UNIMUSCULOSA. TRIDACNA. MODIOLA.] Class.--_Conchifera._ Second Order.--_Unimusculosa._ "1st family, _Tridacnacea_. _Tridacna_ _Chama_. _Hippopus_ _Chama_, (one species.) "In the first genus we find the great _Tridacna gigas_, the largest and heaviest shell yet known. It sometimes weighs five hundred pounds. The hinge has two teeth, the lunula is open, the valves equal, the ligament exterior. * * * * * "_Mytilacea._ _Modiola_ _Mytilus_. _Mytilus_ _Mytilus_. _Pinna_ _Pinna_. "The greater part of these genera attach themselves to marine substances by a byssus. The _Modiola_ genus are rarely found fixed. The ligament internal, lodged in a marginal gutter. Beaks nearly lateral; hinge without teeth. The genus _Pinna_ is unaltered. Small crustaceous bodies, resembling the crab, are sometimes found in the shells of the _Pinna_. * * * * * [Sidenote: PINNA. PERNA. AVICULA.] "_Malleacea._ _Crenatula_ Rare and little known. _Perna_ _Ostrea_. _Malleus_ _Ostrea_, hammer-oyster. _Avicula_ _Mytilus_. _Meleagrina_ _Mytilus_.