Pinna

Pinna, or the "Peabl Mussel," is one of the genera of mollusca belonging to the class Lamellibranchiata, from which pearls are derived. The pearls are formed by a secretion from the mantle over sand grains or other bodies which get between the mantle and shell and. thus irritate the animal. The shells are sometimes two feet in length and are attached by a beard or byssus, the fibres of which have been woven. The shells of the fossil genus Trichites found in the English oolites are sometimes over a yard in length.