The discovery that mussels showing some features used to identify M. galloprovincialis Lmk. (the so-called “Mediterranean Mussel”) are more widespread in Britain than had previously been supposed, led to a re-appraisal of the distinction between this entity and M. edulis L. It is concluded that shell characters are too variable to afford a distinction between these two mussels: the most reliable separating character appears to be the very dark colour of the mantle edge in mussels from the Mediterranean.
Curved and ungulate forms of British mussels are described and it is concluded that these are merely a form of M. edulis—probably the outcome of old age or slow growth. These ungulate mussels share similar shell characteristics with some populations of M. galloprovincialis; also a very dark mantle edge may occur to some extent in populations of otherwise normal M. edulis. This intermingling of characters lends support to the taxonomic conclusion that M. galloprovincialis Lmk. cannot be regarded (on present evidence) as more than a race or subspecies of M. edulis L. In certain circumstances it appears to be impossible to distinguish between the two entities.