1897.] Biological Studies in Massachusetts. 1021 
Chlorophyceze and Infusoria; two of Diatomaceæ, Chloro- 
phycez and Cyanophycee ; two of Diatomacee, Cyanophycee 
and Infusoria. In two ponds all four classes are found in 
large numbers. There is but one pond where the organisms 
never rise above 100 per c.c.; there are sixteen where no 
class of organisms shows numbers greater than 500 perc. c. 
For the purpose of determining whether the depth of the 
pond exercises any important influence upon the growth of the 
organisms the following table has been compiled. Ponds re- 
ported as having a maximum depth of more than 30 feet have 
been called “deep ponds,” and those having a less maximum 
depth “shallow ponds.” This arbitrary dividing line was 
selected after considering the relation between the maximum 
and the average depth, the temperature changes at various 
depths, and the depth to which the water is kept in circulation 
by the wind. Ina general way it may be said that the shal- 
low ponds are those in which the water is kept in vertical cir- 
culation by the wind over the greater part of their area and 
for the greater portion of the year, while the deep ponds ex- 
hibit the phenomena of stagnation over a considerable portion 
of their area. 
TABLE No. 6. 
Number of Ponds 
Depth Number per c. c. 
Diato- Chloro- Cyano- n 
maces. phycee. | phycece. Infusoria. 
Often above 1000 per c. c. í 
Deep Occasionally above 1000 per c. c. 
Deep Usually between 100 & 500 per c. c. 
Deep Always below 100 per c. e. 
Shallow | Often above 1000 per c. c. 
Shallow | Oceasionally above 1000 C. 6. 
Shallow | Usually between 100 & 500 per c. c. 
Shallow | Always below 100 per c. c. 
There are 16 deep and 41 shallow ponds. Of the deep 
ponds 63 per cent at times have growths of the Diatomaceæ 
above 1000 perc. c., while of the shallow ponds 54 per cent 
have such growths. There are no deep ponds where the 
Diatomacee are lower than 100 per c. c., while 15 per cent 
of the shallow ponds have them lower than that figure. It 
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