468 Wisconsin Academy of Sciences, Arts, and Letters. 
ing to size, and these groups were enumerated separately. They 
were recorded as small, in which the length of the shell varied from 
1.5 to 3.5 millimeters, medium with a length of 4 to 6.5 millime¬ 
ters, and large or those having a length of 7 millimeters or more. 
In these 134 samples there were 3,949 individuals, of which 3,343 
were recorded as small, 473 as medium, and 133 as large. Stated 
in percentages of the total number, they were respectively 84.7 
per cent., 12 per cent., and 3.3 per cent. On the basis of these 
figures, it appears that only a relatively small percentage of the 
individuals reaches what may be called full size, that is, the stage 
in which they were classed as large. This seems to indicate a 
rather high rate of mortality. 
While only 33 individuals per thousand reach what may be 
called complete maturity, a much larger proportion become sexu¬ 
ally mature. In addition to the large ones, those classed as medium 
in size were sexually mature, and so were some of the larger ones 
belonging to the class designated as small. This provision for 
early sexual maturity tends to prevent any decrease in the Pisidium 
population which might result from such a high death rate. The 
mortality indicated above does not include that which takes place 
before the one-millimeter stage is reached, and this would doubt¬ 
less make an appreciable addition to the above number. There 
does not seem to be any special reproductive period, since embryos 
in different stages of development are present in the various indi¬ 
viduals at all seasons of the year. This habit of continuous re¬ 
production may be a further provision against undue decrease in 
numbers. 
It has already been stated that there is no free oxygen below a 
depth of 20 meters for a period of about two and a half months 
during the summer. Pisidium seems to pass this interval in a 
quiescent or dormant state; specimens have been kept under obser¬ 
vation for a month or more, and no evidences of any activity what¬ 
ever were detected as long as anaerobic conditions obtained. But 
when placed in aerated water they soon resumed their normal ac¬ 
tivities. 
Several gravimetric determinations were made for the purpose 
of ascertaining the average weight of the individuals belonging to 
the three classes into which this form was separated on the basis 
of size. In some instances the animals were removed from their 
shells, both were dried and weighed separately, and then ashed. 
