The taxonomic confusion surrounding Pisidium (Bivalvia, Sphaeriidae): the possible birth of a new taxon

The presence of Pisidium shells with an appendiculum, presenting specific morphological characteristics not yet described has been observed for several years. Although their external shell traits appear similar the ligament-pit of these individuals differs: it is either long and narrow as in P. supinum, or shorter and thicker as in that of the ponderosa form of P. casertanum and would therefore belong to one or the other of these two species. They could also be the result of hybridisation and constitute a new taxon.