Fig. 149. Tridacna squamosa (Lamarck).
Fig. 150. Tridacna squamosa, on the inside (Lamarck).
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.
Fig. 151. Unio littoralis (Cuvier).
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.