The shell of Anodonta and of all Lamellibranchiata is bivalve. The two valves correspond to the two sides, right and left, of the body, and they resemble one the other. The shell is therefore equivalve. Each valve presents a short straight margin, the hinge-line, along which it is united to its fellow, and which coincides very nearly with the whole of the dorsal aspect of the animal, whilst the long curved margin coincides with its ventral aspect. The external surface is marked by concentric lines parallel with the margin and usually considered to mark distinct periods of growth - a point which cannot be regarded as certain. The areae inclosed by the lines diminish progressively in size. The smallest area corresponds to the original shell. This region of the shell is sometimes remarkably prominent and is known as the beak or umbo. If a line be drawn to the ventral margin from the centre of the umbonal region and perpendicularly to the hinge-line, it divides each valve into a smaller anterior and a larger posterior portion. Hence each valve is inequilateral. Those Lamellibranchiata which move from place to place by means of their distensible foot, move invariably with the anterior part of the shell-valves foremost, and never in the reverse direction.
The shell of the Brachiopoda is also bivalve: but it contrasts with that of Lamellibranchiata in several points: the valves are dissimilar, i. e. the shell is inequivalve: they are divisible into symmetrical right and left halves, i. e. are equilateral: and finally one valve is dorsal, the other ventral according to the usual view. At any rate they are not right and left as in Anodonta. The larger or ventral valve is perforated by a peduncle of attachment, and is uppermost in the natural position of the Brachiopod.
The shell of Anodonta surpasses in size the shells of all other European fluviatile bivalves. It also shows with remarkable distinctness the three layers of which the Lamellibranch shell is usually composed - the epicu-ticula or periostracum, the prismatic layer and the layer of nacre or mother-of-pearl. The epicuticula is purely organic and is composed of conchiolin like the organic substratum of the rest of the shell. It is laminated, and the ridges on the outer surface of the shell are formed by it alone. It constitutes the free border of the shell which is flexible in the natural condition. The prismatic layer is calcified and is visible as a dark border on the inner surface of the shell close to the margin. With a lens it has a shagreen-like appearance due to its division into minute polygonal spaces. The calcareous matter is in the form of more or less regular prismatic needles. The nacre covers the whole inner surface of the shell within the dark border mentioned above. It is iridescent in appearance owing to the diffraction of light by the irregular free edges of the many delicate calcareous lamellae which enter into its composition.
The calcareous lamellae alternate with organic layers.
An elastic ligament unites the two valves along the hinge-line. It serves to open the shell and is antagonised by the two adductor muscles. See next preparation.
The inner surface of each valve is marked by three principal muscular impressions, as they are termed, two anterior and one posterior, placed somewhat dorsally. Of the two anterior impressions, the larger is due to the attachment of the anterior adductor muscle, as well as of the anterior retractor of the foot; while the smaller, placed nearer the free margin of the shell and more dorsally, corresponds to the protractor of the foot. The larger portion of the posterior impression gives attachment to the posterior adductor muscle, but its irregular process, extending dorsally towards the hinge-line, denotes the point of attachment of the posterior and larger retractor of the foot. A continuous line extends between the impressions of the two adductors parallel to the margin of the shell and at some little distance from it. This is the pallial line. It gives attachment to a series of muscular filaments which extend outwards into the free edge of the mantle, and are attached to the spot where the epicuticula commences. The epicuticula being flexible, contraction of these muscles brings the edges of the epicuticular layers of both valves into firm contact, and at the same time retracts somewhat the free edges of the mantle lobes.
As the pallial line describes an even curve throughout its whole extent, Anodonta is said to be integripalliate.
Muscles pass up from the foot and are attached to the ridge which borders the ligament internally, and in shells with hinge-teeth (infra) to the teeth, which are merely developments of the ridge present in Anodonta. A muscle also passes across from the ridge of one to the ridge of the other valve. And small muscular bundles are attached also to the inner part of the ligament
The valves of the shell are inequivalve in the Ostreidae, one valve being smaller than the other. Each valve is nearly equilateral in some of the Pectens, e.g. Pectunculus. It is a rare thing for the anterior portion of the valve to be larger than the posterior. Anodonta, like some of its immediate congeners and some of the oldest Lamellibranchiata, geologically speaking, is devoid of' 'hinge-teeth' on the inner aspect of the valves. The hinge-teeth consist of 'cardinal-teeth' placed below the umbones: of 'anterior lateral teeth' in front of the 'cardinal' teeth and 'posterior lateral' behind them, and below the ligament. Unio, which belongs to the same family as Anodonta, and, like it, inhabits the freshwaters, possesses anterior and posterior lateral but no cardinal-teeth. The presence of these interlocking teeth gives a dissimilar appearance to the two valves when viewed from within. The variations in the character and arrangement of the hinge-teeth have furnished recently a basis for the classification of Lamellibranchiata (Neumayr).