Stiff Pen Shell. Large but delicate, growing to a length
of 1 ft. Shell wedge shaped in outline - dorsal margin straight, ventral
rounded, and posterior gaping. Valves decorated with about 15 rounded and
slightly elevated ribs, which fade near breaks. Highly elevated tabular
spines adorn the ribs, particularly near margins. There is a strong, silky
byssus. Colour of shell is purplish black.