The piddock has a thin, brittle shell that is similar in shape and sculpturing on both sides. The beaks (tips) of each valve are in the anterior half and turn downward and inwards. The whole shell appears in outline to be elongate and oval with a maximum length of ca 6.35 cm. The surface of the shell is sculptured with numerous concentric lines crossed by >40 radiating ribs, which in the anterior half of the shell are large and coarse, bearing prominent spines. Outer surface of shell is off-white or fawn in colour with a dark brown periostracum. The ligament (which opens the shell) is external and prominent, stretching a quarter of the way between the beaks to the posterior margin. Petricola pholadiformis may be confused with Barnea candida, the native white piddock of the Pholadacea (Tebble, 1976).Common names
Petricola pholadiformis may also be known as the 'false angel wing'.
 Method of introduction and spread
Petricola pholadiformis is a non-native, boring piddock that was unintentionally introduced from America with the American oyster, Crassostrea virginica, not later than 1890 (Naylor, 1957). Rosenthal (1980) suggested that from the British Isles, the species has colonized several northern European countries by means of its pelagic larva and may also spread via driftwood, although it usually bores into clay, peat or soft rock shores.
In Belguim and The Netherlands Petricola pholadiformis has almost completely displaced the native piddock, Barnea candida (ICES, 1972). However, there is no documentary evidence to suggest that native piddocks have been displaced in the British Isles (J.Light & I.Kileen pers. comm. to Eno et al., 1997).