The Pinnæ often grow to a large size; they are sometimes found standing erect in the smooth-water bays, with the base of the shell uppermost, but generally affixed by the byssus to rocks and other sub-marine bodies. The filaments that compose the byssus are so tough and strong that the shells are not easily detached.

Shell fibrous, fragile, regular, equivalve, longitudinal, triangular, base gaping and as if truncated; summit pointed and straight; hinge dorsal, longitudinal, linear, and without teeth; marginal ligament occupying almost the whole of the dorsal edge of the shell; one very broad muscular impression behind a trace of the anterior in the summit.

Pinna rudis.

P. flabellum.

P. seminuda.

P. angustina.

P. nobilis.

P. squamosa.

P. marginata.

P. muricata.