|Pinnidae, the pen shells, have thin, fan-shaped, translucent shells. The outer shell may have radial ribs and or small scales or spines. The inside has a shiny, iridescent part called the nacreous layer (or nacre). The hinge is long and straight. They live partially buried in muddy, sandy or shelly substrates, with the rounded end exposed, by about 1 inch, and agape. Byssal threads are used to attach to the substrate. The shells are very fragile and are difficult to find intact while beachcombing, let alone get it home without breaking it. There are 5 species found in Texas.