Atrina rigida:

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.