Aided by this silky tuft, the animal attaches itself to the rocks, and suspends its weighty shell from them. If it is intended to remove those attached to the sides of the rock, it is necessary to cut the cords of the tendonous byssus, by which it is held suspended, with a hatchet. All the species are inhabitants of the Tropical seas. The _Tridacna_ _gigas_ is a native of the Indian Ocean. The flesh, though coriaceous, and by no means of an agreeable flavour, is a great resource to the poor Indians. The accompanying representations of _Tridacna squamosa_ (Figs. 149 and 150) will convey a general idea of the genus. Our fourteenth family, Hippuritidæ, is entirely fossil; but the fifteenth, Chamidæ, of which the best example is the rugose genus Charina, is widely distributed in tropical seas. The very numerous division of shells called Asiphonidæ, possesses animals without respiratory siphons. The shells we shall now describe belong to the sixteenth family, Unionidæ, which contain Iridina, Anodon, and Unio. The pond mussels, _Anodon_, are found in lakes, rivers, and seas of almost every region of the globe. Their shells are rounded or oval, generally very thin, regular, and equivalve, not gaping, the hinges without teeth, whence their name, from the Greek, ὀδότοϛ, _without teeth_. These shells are nacred inside, and generally smooth. The _Anodon cygnea_ (Fig. III., PL. XVI.) is broad, deep, and light, and is sometimes employed for skimming the cream off milk. The genus is divided into many groups, the principal forms of which are represented in PL. XVI. The river mussels, _Unio_, are, like the Anodon, found in the muddy bottoms of all countries. The animal resembles the Anodon, but the shell presents a toothed hinge. The lower face of the valve is nacrous, but shaded with purplish violet, copreous, and iridescent; the anterior face is of a green colour, which varies from tender to blackish green. [Illustration: Fig. 151. Unio littoralis (Cuvier).] Among the species found in European seas may be noted the Rhine mussel, a large species, the nacre of which is employed for ornamental purposes. _Unio littoralis_ (Cuvier), represented in Fig. 151, and the painter's mussel, _Unio_ _pictorum_ (Fig. 152), employed in the arts to contain certain colours. Those known as the river mussels are leathery, of an insipid taste, and scarcely eatable: the finest species are found in the great American rivers. [Illustration: PLATE XVI.--Anodonta. I. Anodonta angulata. (Lea.) II. Anodonta ensiformis. (Spix.) III. Anodonta cygnea. (Linn.) IV. Anodonta magnifica. (Lea.) V. Anodonta anserina. (Spix.) VI. Anodonta latomarginata. (Lea.) ] [Illustration: Fig. 152. Unio pictorum (Linnæus).]