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BULLETIN OF THE UNITED STATES FISH COMMISSION. 
another to sub-Carboniferous and Trenton. Two of the Wisconsin papers mention 
limestone underlain by Potsdam sandstone and associated with St. Peter’s sandstone. 
Of course the rocks of the Mississippi Valley are for the most part well known, and 
the particular horizons here mentioned, so largely confined to the earlier Paleozoic, 
can have no special significance in the present connection, as only the chemical com- 
position of the rocks could affect the abundance of the Unios, if, indeed, the limestone 
theory be as important in this respect as is generally believed. It may be observed 
here that in several papers which make no mention of limestone or specify other rocks 
instead (Illinois, Michigan, and Texas, sandstone; Florida and Mississippi, sands and 
clays) there seems no dearth of Unios in the streams and lakes. 
Out of 107 papers which respond definitely to the third inquiry, as to the abun- 
dance of mussels, 10 describe the shells as at present very abundant, 47 as plentiful, 
36 as scarce, and 4 as absolutely exterminated, while 34 papers refer to the fact of 
diminished and diminishing numbers within a few years past, some of them with great 
emphasis. Three Tennessee papers estimate the present numbers as reduced to one- 
tenth of what they were ten years ago, and in a number the same general fact is 
stated — of former abundance and present rarity — and attributed to the pearl-hunting 
destruction of recent years. Several papers say that the shells are now scarce in small 
streams and the shallower parts of larger ones, while still abundant in deep water 
and where the currents are strong. The answers in detail are as follows : 
General abundance of mussels. 
In response to the inquiry as to the form, size, and position of the beds, the answers 
are very various, indicating much diversity of conditions, depending evidently on the 
species of shells and size and character of the streams. A Wisconsin report states the 
river to be “ nearly all mussel-bed for 100 miles.” A Tennessee report states that shells 
lie scattered over the bottom and not in beds, and reports from Iowa and Massachusetts 
make similar statements. Some 39 papers give estimates of the size of the beds, 
