* 365 
than was practicable, with also an annoying tendency to in-leakage of 
outside water at the bottom. 
Valuation—tThe determination to undertake a valuation of the bot- 
tom invertebrate populations that come within the feeding horizon of our 
ordinary bottom-feeding fishes in terms of pounds pet acre was made 
some time after the conclusion of our field work in 1915, and has been 
carried out on a basis of estimated average-sized specimens of the va- 
rious species as they ran in a relatively small number of typical midsum- 
mer collections weighed after more than a year’s preservation in alcohol 
and formalin. An average correction of 25 per cent. for loss in weight 
in alcohol (on a base of body weight only for Mollusca, and on a base 
of gross weight for other groups) has been made, after a limited number 
of experimental weights, in the preserved and the fresh state, of a few 
snails and insect larvae. The final valuation figures, so far as they in- 
clude insects, their larvae or other immature forms, worms, or Crustacea, 
represent gross rough weights, but in the case of the Mollusca (Gastrop- 
oda, Sphaeriidae, or young Unionidae) represent the body weight only, 
after deduction of shell weights at rates determined separately for each 
species by actual weighing. Sponges, Bryozoa, and other smaller in- 
crusting invertebrates are not included in the valuation figures ; as are not 
also crayfish or pearl-button mussels, except the young. 
Acknowledgments—For many of the hydrographical and physical 
data we are indebted to the U. S. Army Engineers’ survey of 1902—1905 
(House Document 263, 59th Congress, 1st session, and accompanying 
charts) ; as well as to Alvord and Burdick’s recent excellent report 
(1915) on the Illinois River and its bottom-lands; and, in a lesser de- 
gree, to the Report of the Legislative Committee on submerged and shore- 
lands (1911). Thanks are also due Dr. Edward Bartow, Chief of the 
State Water Survey, for his interested cooperation in obtaining the san- 
itary chemical analyses of river waters in 1914; and to Prof. S. W. Parr 
for supervising the analysis of the bottom mud samples taken that year 
and the year preceding. To Mr. Charles A. Hart is owing a special debt 
for his assistance in the determination of much of the more unfamiliar 
biological material of the earlier collections, taken in the preliminary 
field work of July-September, 1913. Mr. F. C. Baker has contributed 
both facts and opinions that have made possible rough valuations, for 
comparison with our own, of the littoral bottom fauna areas of the lower 
south bay of Oneida Lake, New York, reported upon by him in two very 
interesting and valuable papers in 1916 and 1918. To these two papers 
and to Dr. C. G. Joh. Petersen’s several recent contributions on the valua- 
tion of sea-bottom off the Danish coast (Reports of Danish Biological 
Station, 1911-1918), I owe not a few ideas which have cast illumination 
in more or less dark places. The general plan into which the present 
piece of work is intended to fit, the directing force behind it, and the sup- 
ply of means and general suggestions as to methods for its execution, 
have been the work and care for many years of the Chief of the Natural 
