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3 shells appear lopsided. The short end is anterior and the long end is posterior. "Pseudocardinal" means "false" cardinal. So, if these are "false" cardinal teeth, then what are "true" cardinal teeth? Many marine mussels (and some freshwater mussels such as the Asian clam) are symmetrical and at the beak position there are a set of true cardinal teeth. Flanking the cardinal teeth on both sides are two sets of lateral teeth. Freshwater mussels have only one set of lateral teeth and, what should be the cardinal teeth, are set over on the side. Since they aren't the topmost, they have been called "false" cardinal or pseudocardinal. These pseudocardinals are a set of short, blunt teeth on the anterior end of the beak. On the other or posterior side are a set of long-thin ridges which are the lateral teeth. In most species, there will be two lateral teeth in one valve and one lateral tooth in the other valve. On the surface of the beak will often be a series of raised ridges which form a series of lines or loops that are unique to each species and are important in their identification. These ridges, if present, are called the beak sculpture. The core of a mussel shell is composed of calcium carbonate extracted from the water. The mussel grows by the addition of material on the edge and on the interior surface of the shell by the mantle. The mantle lines the interior of the shell, wrapping around the internal organs and is attached to the shell at the pallial line. The exterior of the shell is protected by a layer of tissue called the periostracum. This protects the core from abrasion and from being dissolved by acidic water. The exterior is often marked with concentric rings. These represent periods when growth has stopped (such as in winter) and are usually interpreted as annual growth rings. But growth can stop and rings may form during periods of stress such as drought or a physical disturbance. The interior of the shell is covered by a dense layer of calcium carbonate called the nacre or mother-of-pearl. The color of the nacre can vary from white to pink to deep purple and is often iridescent. At the position of the beak there is a beak cavity which can be deep or shallow and can help identification. At each end of the mussel shell will be circular scars which show where the anterior and posterior adductor muscles were

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