Pinna similis

Lower Jurassic, Lias Group, Waterloo Mudstone Formation

Drumnagreagh, Co. Antrim

K28656

85 mm long

A very distinctive, narrowly triangular bivalve; viewed end-on it has a diamond-shaped cross section. Occasional examples may be more than 300 mm long!

It is rather similar in general shape to Isognomon but the latter is much flatter, without the diamond-shaped cross section so characteristic of Pinna.

Pinna lived vertically in the mud, with only the gaping end of the valves visible on the sea floor.