Wikipedia
Mucrospirifer is a genus of extinct brachiopod in the class Rhynchonellata (Articulata) and the order Spiriferida. They are sometimes known as "butterfly shells". Like other brachiopods, they were filter feeders. These fossils occur mainly in Middle Devonian strata.
The biconvex shell was typically 2.5 cm long, but sometimes grew to 4 cm. The shell of Mucrospirifer has a fold, sulcus and costae. It is greatly elongated along the hinge line, which extends outward to form sharp points. This gives them a fin- or wing-like appearance. The apex area (umbo) of the pedicle valve contains a small fold for the pedicle. Mucrospirifer lived in muddy marine sediments, and were attached to the sea floor via the pedicle. The shell sometimes looks like two seashells stuck together.