296 
imately two months’ duration, reveals an almost complete ex- 
termination of the plankton, the amount given in the table,. 01 
cm.*, being only an expression for an amount beyond the reach 
of our methods of measurement. As shown in Plates VIII. 
and IX., there was prolonged and quite stable low water from 
Oct. 15, 1894, till the flood of the last week of February of the 
following year which carried away the ice. The concentration 
of sewage under such conditions was shown by the stench of 
the water, by the departure of fish into tributary backwaters, 
and by the death of many not escaping. Unfortunately no 
chemical analyses of river water at this season are now availa- 
ble, and the chemical conditions can only be inferred from 
those in later years at times of briefer ice blockade, higher 
water, and presumably less contamination. For example, in 
January, 1898, following the low water of 1897, we find under 
ice of three weeks’ duration (Pl. XI., XII, and XLIV., XLV.) 
great excess of freeammonia and chlorine, and high albuminoid 
ammonia, organic nitrogen, and oxygen consumed—all, in- 
dices of contamination. The ice sheet upon a contaminated 
stream must also profoundly affect the equilibrium of oxygen 
and carbon dioxide dissolved in the water, and thus directly 
influence the life of all constituents ofthe plankton. It is there- 
fore not surprising that these unusual conditions should ex- 
terminate all but the most resistant members of the plankton. 
The catch consisted almost entirely of flocculent debris (zo- 
ogloee?) with a few minute filaments of bluish green alga of 
uncertain affinities, while the usual plankton was represented 
by only 43 individuals, representing 14 + species, as follows. 
Protozoa: 
Diffugia sp. (deformed?).......... il 
Epistylis sp., heads............... 3 
Carchesium lachmanni, head....... 1 
Ciliate, indeterminate............ 1 
Heliozoan: :. fo Skee eee ea kee 1 
