210 College of Forestry 
mollusks, and crustaceans.’’ In the table the mollusean food 
is placed at one per cent., as not enough data is at hand to 
provide a more definite ratio. 
Juday (1907, pp. 164-171) examined 394 trout stomachs 
representing six species, from ‘Twin Lakes, Colorado. The per- 
centage of molluscan food was .22. In two species the mol- 
lusecan content was 1 per cent., or more, viz.: Landlocked 
Salmon (Salmo sebago) of which one specimen contained 1 
per cent., and the Rainbow Trout (Salmo trideus shasta) in 
which one specimen contained 40 per cent. of molluscan food. 
The variation in the character of the food, both in the age 
of the fish and in the different localities, is well indicated. 
D. .THE PLANKTON. 
Reference has been made on previous pages to the plankton 
and its use especially by young fishes. The plankton is the 
floating population of the water and consists of alge, diatoms 
(plants), protozoans, rotifers, Entomostraca, Chironomus and 
a few other insect larvee, and other small animals. Among 
the plankton are such mollusks as Ancylus, Amnicola, and the 
smnall Planorbes (Planorbis parvus) which become loosened 
from plants and float in the water (Kofoid, 1908, p. 287). 
They may often be found gliding on the under side of the sur- 
face film of the water. 
Among the bivalves or peleeypods, the young or glochidia 
have been observed in the plankton and are perhaps some- 
times eaten, though more often they find lodgement in the 
gills or on the fins where they carry on a stage of their 
development. The glochidia of three species were observed 
in the plankton of the Illinois River by Kofoid (pp. 
287-288) as noted below: 
Anodonta carpulenta. Observed in October—April and November— 
March. 
Lampsilis anodontoides. Observed in September—December and June- 
Cee confragosus? Observed in December. Identification doubtful. 
It is unusual, it would seem, for any amount of glochidia 
to form a part of the plankton, because when discharged 
