1897.] Biological Studies in Massachusetts. 1017 
subject as they show the relative abundance of the different 
classes of organisms in some of the important surface water 
supplies of Massachusetts, together with some of the elements 
of the sanitary chemical analyses. 
For the purpose of this comparison 57 ponds and reservoirs 
were selected where monthly examinations, both chemical and 
biological, have been carried on for a number of years by the 
State Board of Health. The results of these examinations were 
carefully studied and the ponds (which, for convenience, we 
may consider to include lakes, ponds and storage reservoirs) 
divided into groups as shown in Table 4. 
The first two columns in this table give the names of the 
ponds and the cities which they supply. The third gives the 
depth of the pond, whether shallow or deep. The next four 
columns show the relative abundance of the four most impor- 
tant classes of organisms, namely, Diatomaceæ, Chlorophycee, 
Cyanophycez and Infusoria. The four groups are charac- 
terized as follows: and the group to which each pond belongs 
is indicated by a number.: 
Group I. Number of organisms often as high as 1000 per 
C, C. : 
Group II. Number of organisms only occasionally as high 
as 1000 per c. c. 
Group III. Number of organisms ordinarily between 100 
and 500 per c. c. 
Group IV. Number of organisms never above 100 per c. c. 
These figures refer not to the numbers present in the aver- 
age sample of water, but to the numbers during the season of 
maximum growth. The boundaries of the groups were not 
sharply defined, and in a number of cases it was hard to tell 
whether a pond should be classed in group II or III. The 
last five columns show the ponds divided into classes ac- 
cording to some of the elements of the chemical analysis, 
namely, color, excess of chlorine, hardness, albuminoid ammo- 
nia (in solution), free ammonia and nitrates. In each case 
four classes are given, division being made according to the 
schedule given at the bottom of the table. 
