THE ECONOMIC MOLLUSCA OF ACADIA. 71 



the sea-shore must know it well. There is only one common Mollusc at 

 .all 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 

 Tinob around which the concentric lines of growth of the shell are 

 arranged ; this is at the extreme end in J/. ednJis, but set a little back 

 near one margin in Modiola modiolus. Its lack of raised ribs distinguishes 

 it from M. plicahda. 



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 pellucidiis is given. Within, the 



Fig. 11. — My tibus edulis. Natural Size. 



«hell is dark-yiolet, silver and while, 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 by 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 



