"The body is soft, without joints, without head or eyes; it is wrapped in a mantle or tunic. The mouth, always hidden in the tunic, is merely an opening to admit food, without jaws or teeth. The shell is always bivalve; the valves united by a hinge or a ligament; sometimes there are accessory pieces to the valves. [Sidenote: CONCHIFERA. UNIMUSCULOSA.] "Some of the _Conchifera_ are furnished with a kind of foot, which enables them to move with their shells, to draw out fibres by which they fasten themselves to marine bodies. The muscles that fasten the animals to their shells are thick and strong; their use is, to close the valves by contracting; when the muscle is relaxed, the elastic ligament is sufficient to open them. The _Conchifera_ are all aquatic; some inhabit fresh water, the others dwell in the sea. "The class contains nineteen families and two orders. The first order, _Bimusculosa_, contains thirteen families. The first includes genera that you will scarcely expect to find among the bivalve shells. * * * * * "1st Family, _Tubicolaria_, contains, _Aspergillum_, Watering-pot _Serpula._ _Clavagella_ ------- _Firtuluna_ _Septaria_ _Serpula._ _Teredina_ Fossil genus. _Teredo_ _Teredo._ "_Aspergillum Javanum_ is a rare and curious shell from the Indian seas, (Plate 9.) The whole family is remarkable, and was referred, you perceive, to a very different order. _Clavagella_ was till lately considered as existing only in a fossil state. The researches of recent travellers have discovered _Clavagella_ at Port Jackson, in Australia.[A] There is a specimen in the British Museum. The valves are enclosed in the tube. [A] See Penny Cyclopædia, article Clavagella. "The valves of _Teredo_ are noticed as forming part of the Linnæan generic character, you will recollect. Lamarck considers them as true _Conchifera_. In many specimens of _Teredo_ the valves are wanting, and the tube only remains. * * * * * [Sidenote: PHOLADARIA.] "The family _Pholadaria_ contains, _Pholas._ _Pholas_, stone-piercer. _Gastrochæna_ _Pholas_ and _Mya_. "Notwithstanding the accessory pieces of the hinge, _Pholas_ is placed among bivalve shells, the essential character of which is to have two valves united by a hinge. The _Pholas_ has a foot or strong muscle, very thick and short. In the next genus, composed of _Pholas hians_ and _Mya dubia_ there are no secondary valves. "Allied to this family is _Xylophaga dorsalis_, a curious shell. One specimen has been lately found at Gravesend, upon a stick. * * * * * [Sidenote: SOLENACEA. MYARIA. MACTRACEA.] "_Solenacea_ includes _Solen_ _Solen._ _Panopæa_ _Mya._ _Glycimeris_ _Mya._