Pinna rudis, the rough pen shell or spiny fan-mussel, is a species of bivalve mollusc in the family Pinnidae.

[2][4][5][6][7][8] The rough pen shell has a pair of very fragile, long, triangular, wedge–shaped valves, covered with large, protruding scales arranged in quite regular rows.

The spiny fan-mussel lives with the pointed anterior end of its shell vertically anchored to rock or firm sediment by numerous byssus threads.

P. rudis may be confused with juveniles of Pinna nobilis, but the former shows a more triangular and robust shell, with fewer and larger protruding scales.

Lastly, the adults of P. nobilis easily exceed the size of P. rudis and totally lose the protuberances on the surface of the shell.