(Sars, 1866)
Description:
Carapace medium to large, 0.65-0.7 mm long; dimorphic, males smaller and proportionally more elongate than females. Greatest height slightly in front of mid-length; dorsal margin broadly arched, both ends rounded but posterior more tapered. Moderately inflated in dorsal view, greatest width in front of midlength, rounded posteriorly, acutely tapering anteriorly. Narrow marginal flange anteriorly, and to a lesser extent posteriorly, in both valves. Male copulatory appendage distinctive.
Habitat:
It has been found on a variety of substrates in sublittoral marine environments, though never abundandy.
Remarks:
P. trigonella has often been confused with P. pirifera; P. trigonella is smaller, more elongate, and its greatest height and width are further forward than in P. pirifera (see also Remarks on P. pirifera ).
Distribution:
A NW European species; Scandinavian and British coasts, Mediterranean records need to be checked. Because of the common confusion of this species with P. pirifera the true extent of its distribution is difficult to assess.