In an earlier paper (Gibbs, 1973) it was shown that in populations of Golfingia elongata (Keferstein) in the Plymouth area, a significant proportion of individuals possess fewer retractor muscles than is typical for the species. These individuals have three or two retractors instead of the normal four (= subgenus Golfingia) but otherwise are typical G. elongata in all characters. A morphologically similar species G. rimicola Gibbs is also common in the area and this species differs in possessing only two retractor muscles (= subgenus Phascoloides). One possible reason for the presence of aberrant individuals with an intermediate number of retractor muscles is hybridization; if the spawning seasons of the two species overlap in time, then cross-fertilization could be a feasible explanation for this anomaly.