Pinnidae is a family of large saltwater clams sometimes known as pen shells. They are marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 bivalve molluscs in the order Pterioida

Pterioida

Pterioida is an order of large and medium-sized saltwater clams, marine bivalve mollusks. It includes five families, among them the Pteriidae -Families within the order Pterioida:* Isognomonidae* Malleidae* Pinnidae...

.

The shells are fragile and have a long and triangular shape, and in life they are anchored in sediment using a byssus

Byssus

Byssus means both a silky filament by which certain molluscs attach themselves to hard surfaces, and a rare fabric, also called sea silk and its fibre source.-Word:...

. The shells have a thin inner layer of nacre

Nacre

Nacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....

 in the part of the shell which is near the umbos (the pointed end.)

The family Pinnidae includes the fan shell, Atrina fragilis

Atrina fragilis

The fan mussel, Atrina fragilis, is a large saltwater clam or marine bivalve mollusc in the family Pinnidae, the pen shells.The fan mussel is one of the largest European bivalve molluscs. It is one of the rarest species of marine mollusc in the United Kingdom .The shell of the fan mussel tapers to...

, and Pinna nobilis

Pinna nobilis

Pinna nobilis, common name the "noble pen shell" is a species of pen shell, a large marine bivalve mollusc in the family Pinnidae, the 'pen shells'....

, the source of sea silk

Sea silk

Sea silk is an extremely fine, rare and valuable fabric produced from the long silky filaments or byssus secreted by a gland in the foot of several bivalve molluscs by which they attach themselves to the sea bed....

. Some species are also fished for their food value.

"“The Pinnidae have considerable economic importance in many parts of the world. They produce pearls of moderate value. In the Mediterranean area, material made from the holdfast or byssus of Pinna nobilis Linné has been utilized in the manufacture of clothing for many centuries: gloves, shawls, stockings and cloaks. Apparel made from this material has an attractive golden hue and these items were greatly valued by the ancients.
Today, pinnidae are eaten in Japan, Polynesia, in several other Indo-Pacific island groups, and on the west coast of Mexico, In Polynesia, the valves of Atrina vexillum are carved to form decorative articles, and entire valves of larger specimens are sometimes used as plates.”

Genera

• Atrina
  Atrina
  Atrina is a cosmopolitan genus of bivalve molluscs characterized by elongated, wedge-shaped shells, distinguished from the genus Pinna by the lack of any grooves in the nacreous lining of the shell, and by the central positioning of the adductor scar....
  
  Gray, 1842
• Pinna
  Pinna (genus)
  Pinna is a genus of pen shells. It is a cosmopolitan genus of bivalve molluscs characterized by elongated, wedge-shaped shells which most commonly stand point-first in the sea bottom in which they live, anchored by a net of byssus threads....
  
  Linnaeus, 1758
• Streptopinna
  Streptopinna
  Streptopinna is an Indo-Pacific genus of bivalve molluscs characterized by a roughly triangular outline.There is only one species, the Streptopinna saccata....