Fenestella is classed as a Polyzoan (many animals) although the term Bryozoan (moss animal) is still favoured in the U.S.A. Like a coral, it is composed of a calcareous skeleton housing many individual organisms.
Polyzoans come in many shapes: rods, sheets, branching structures and spirals. They were important contributors to reef building during the Palaeozoic Era.
Fenestella is usually found only as fragments because of its delicate fan-like structure. This specimen is unusual in being over 10cm accross. Little detail can be seen on the large image but the magnification shows the finer detail of the structure.