_b._ The Brachiopoda (_arm-feet_) have two long spiral fleshy arms, or brachia, developed from the sides of the alimentary orifice, are enclosed in bivalve shells, and respire by means of their vascular skin, or mantle. They have not the power of locomotion, but are fixed by a peduncle to other bodies. _c._ The Lamellibranchia (_plated gills_) have also bivalve shells, but their respiration is effected by gills composed of vascular membrane disposed in plates, and attached to the mantle; the _beard_ of the Oyster is the branchial or respiratory apparatus of that animal. These bivalve Mollusca are subdivided into those which close their shells by one adductor muscle, hence called _monomyaria_, as the Oyster; and those which have two muscles, _dimyaria_, as the Cockle. As the impressions left on the shells, by the attachment of these adductor muscles, and by the margin of the mantle, are found as perfect in the fossil as in the recent, they constitute important distinctive characters. Dr. Gray's definition of the respective parts of univalve and bivalve shells is at once clear, concise, and natural, being conformable to the structure of the body of the enclosed mollusk. The _front_ of the shell is the part which covers the head of the animal; the back of the shell is the part which covers the tail; the left and right sides correspond with the same parts of the mollusk. In univalves, the apex of the shelly cone whether it be simply conical or spiral (except in Patella) is over the hinder part of the animal: when the shell is placed on its mouth with the apex towards the observer, the parts of the shell correspond with the position of the person looking at it. [Sidenote: FOSSIL BIVALVE SHELLS.] [Illustration: Lign. 121. Illustration of Fossil Bivalve Shells; _nat._ Petricola Patagonica. _D'Orb._ Interior of right valve, and the same valve with the animal as seen on the removal of the left valve. _a._ Labial Palpi. _b._ Mantle. _c._ Margin of shell. _d._ Branchiæ. _e._ Anal siphon. _f._ Branchial siphon. _g._ Foot. _h._ Retractor muscle of siphons. _i._ Posterior adductor. _j._ Anterior adductor. _k._ Ligament. _l._ Umbo. _m._ Lunule. _n._ Base, or ventral margin. _o._ Anterior side. _p._ Posterior side. _q._ Pallial line. (The length of the shell is estimated from _o_ to _p_, its breadth from _l_ to _n_.) ]