12 
recently described species. In these instances the difficulty lies 
not so much in finding representatives of these closely related 
species, but, rather, in drawing the lines between them and placing 
every individual enumerated in the proper pigeon-hole. To avoid 
this difficulty, the separation was not attempted in every case. 
With the hope that the results would throw some light on the ques- 
tion of seasonal variation, this separation was attempted in the 
genus Brachionus, where the species characters are confined to 
prominent structural features. 
So far as it was feasible, specific distinctions were accepted as 
found, and utilized whenever possible. In the lists and discussions 
which follow, the inclusion of a species does not necessarily carry 
with it the inference that it is regarded by the writer as valid or 
well founded. It merely represents in our enumerations a more or 
less continuous succession of organisms which conform approxi- 
mately to the descriptions and figures of the species designated by 
the name in question. Inferences regarding the rank or validity 
of the species reported will be given whenever the statistical data 
or my observations on the variability of the organism seem to 
afford data bearing on the standing of the species. While not a 
few of the species reported may justly be regarded as synonyms, an 
effort has been made to use only names which represent valid 
species or at least a variety or a seasonal form. 
COMPARISON OF FRESH-WATER AND MARINE PLANKTON. 
The plankton of fresh water is very generally composed of an 
assemblage of organisms, of plants and animals, principally crypto- 
gams and invertebrates. Not all orders are represented, and those 
that do occur vary greatly in the number of their representatives. 
The fresh-water plankton differs from that of the sea in the almost 
universal absence of larval forms, in the smaller number of inverte- 
brate groups represented, and in the smaller size of its component 
organisms. Fresh-water plankton has almost no limnetic ccelen- 
terates, Hydra fusca being the only representative as yet discovered 
in our locality. The absence of the larger Crustacea, of limnetic 
mollusks and worms, and of tunicates and Radiolaria robs limnetic 
life of the diversity found among pelagic organisms of the sea. 
The only larval stages found in our locality are the glochidia of the 
