THE ECONOMIC MOLLUSCA OF ACADIA. 
71 
the sea-shore must know it well. There is only one common Mollusc at 
^11 likely to be mistaken for it — and that is the Horse-mussel, Modiola 
modiolus. But the former is blue-black generally, and the latter is 
chestnut, or rather its epidermis is, and their situations or habitats are 
different, as will be seen by reading the descriptions of both. A good 
mark to separate them is found in the position of the umbo, or raised 
knob around which the concentric lines of growth of the shell are 
arranged ; this is at the extreme end in M. edulis , but set a little back 
near one margin in Modiola modiolus. Its lack of raised ribs distinguishes 
it from M. plicatula. 
This species varies somewhat in color, the variant individuals being 
of a darker or lighter horn-color, and generally showing radiating, 
longitudinal, dark lines. These forms are by some considered to form a 
•distinct variety — to which the name pcllucidus is given. Within, the 
Fig. 11 . — Mytilus edulis. Natural Size. 
•shell is dark-violet, silver and white, and often very beautiful. The 
average length of adults is about three inches, though they grow as great 
as five, and even over eight inches in rare cases in Europe. They vary 
greatly in outline, the nature of the variation of an individual depending 
chiefly upon the conditions under which it has grown. 
It is a fact of no little importance to man, as will be presently seen, 
that the edible Mussels are gregarious. All visitors to the shore remember 
well the great beds, often acres in extent, which are found on the flats, 
especially of estuaries. They never burrow, but instead, fix themselves 
by means of a mass of silky threads which are secreted b} r the foot and 
pass out between the valves. With these they can attach themselves to 
.any surface, sand, mud and gravel, rocks, piles of wharves, buoys, 
bottoms of boats, and, in fact almost anywhere. They prefer, however, 
mixed gravel and mud flats where the water is not quite pure, but has 
some admixture of river water. The mud flats they add to, both by 
