Hitherto the most general and fatal danger to oyster-beds has been the ungoverned extravagance of irresponsible fishers who seek to harvest in the present regardless of the future, but these are gradually being made amenable to restrictive laws as authorities awake to the value of the industry. A greater danger which threatens the unio of American streams, is the pollution of the water by the discharge of the refuse of factories and the sewage of cities into them. A mussel bed will recover in time when denuded by fishers, but sewage and poison kills it out entirely. Although fresh-water pearl-bearing mussels are found in the streams of many countries, only in the United States are they taken in sufficient quantities to make the fishings important as an industry. They are to be found throughout the Mississippi drainage area and in part of that of the St. Lawrence. Few exist on the Pacific coast and those of the Atlantic coast are generally inferior as pearl-mussels. There are many varieties of the unio which yield pearls. Latin names are given by different writers to distinguish them, but as scientists differ in their classifications, the names are not always uniform and are not sufficiently well established to be useful, descriptively, to the general reader. In treating of the various kinds of pearl-bearing unios of the United States therefore in these pages, the common names by which they are known will as a rule be used with the scientific names appended, as revised by the department of mollusks of the United States National Museum. From the times of Roman colonization until now, pearls have been taken from the mussels of British streams. There are three varieties of pearl-bearing mussels in Great Britain: Painter's mussel (U. pictorum), the Swollen River mussel (U. tumidus) and the Pearl mussel (U. margaritifera). The first two occur only in the streams and ponds of England and Wales and the pearls found in them are of inferior quality. The latter inhabits the streams of Scotland and the northern counties of England and to some extent are found in Ireland and Wales also. The shell is oblong, rather flat and heavy and about five and one-half inches long. The exterior surface is rough, and blackish-brown; the pearly interior has a tint of flesh color mottled by stains of dull green. It was from this variety the Perthshire Tay pearls were taken, which gained so much notoriety in the middle of the eighteenth century when some fifty thousand dollars worth were sent to London from this stream in three years.