The Brachiopoda, though presenting a certain outward resemblance to bivalved Mollusca, are quite distinct from this group. They are all marine, and all possess a bivalve shell. They grow attached to rocks (Fig. 1), usually by a horny peduncle or stalk passing between the two valves, or through a foramen in one of the latter; or, peduncle and foramen may be absent, one of the valves adhering by its surface to the rocks; some species of _Lingula_ live in tubes in the sand or mud. They occur at all depths, from shallow water up to 2900 fathoms, but the largest number of species live at a depth of about 350 fathoms. Though found in all seas, the localities whence they have been obtained are comparatively few in number; but specimens are usually congregated in considerable numbers, in places where they do occur. The surviving species of Brachiopods constitute only a small remnant of a group that flourished abundantly in former epochs. There are about 150 recent, and over 6000 fossil species. THE SHELL.—The valves of a Brachiopod shell differ from each other in size and shape, but each valve is in itself symmetrical, _i.e._, similar on each side of a middle line. The valve through which the peduncle passes is termed the _peduncle_ or _ventral valve_ (Fig. 2, A), the other being the _brachial_ or _dorsal valve_. The peduncle valve, which is usually the larger and uppermost, contains the bulk of the viscera; in the higher genera, calcareous bars or loops (Fig. 2, B) attached to the inner surface of the brachial valve form a support for the “arms” of the animal. The inner surface of the valves presents certain markings and depressions where the muscles have been attached (Fig. 6). [Illustration: Fig. 2. _Magellania flavescens._ Australia. Interior of valves. A. Peduncle valve: _f_, foramen for peduncle, below which are the two small deltidial plates; _t_, hinge-teeth; _a_, _b_, _c_, muscle scars. B. Brachial valve, showing the reflected loop for support of the “arms.” ] The shell is constructed of very minute prisms of calcareous substance imbedded in an organic matrix. In _Lingula_ the shell is formed of alternating layers of horny and calcareous substance. The shell-valves are either hingeless, or joined by a hinge in which teeth in the peduncle valve fit into sockets in the brachial valve. The Brachiopoda are primarily divided into two sections, _Inarticulata_ and _Articulata_, based on the absence or presence of a hinge.