46 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
Lake Superior Entomostraca, 1891; Marsh: On the Deep- Water Crustacea 
of Green Lake, 1891; Forbes: A Preliminary Report on the Aquatie In- 
vertebrate Fauna of the Yellowstone National Park, 1893; Marsh: On the 
Cyclopide and Calanide of Central Wisconsin, 1893. (2) 
While it is probable that comparatively little remains to be done 
in the systematic study of North American Copepoda except in the 
west and in mountainous and brackish stations, yet there are few 
more attractive fields than that offered by the study of the biological 
conditions under which these forms appear. The processional ap- 
pearance of several species in the same pool in the spring, the effects 
of stagnation on the organization, and the peculiar heterogenesis or 
polymorphism due to age, are all capable of yielding valuable addi- 
tions to theoretical biology. While much care and attention are 
requisite, and considerable collateral information, yet the Cladoceran 
studies of Professor Weismann afford a suitable model and induce- 
ment. Ina purely systematic paper like the present one no attempt 
can be made to indicate these lines of work. 
FAMILIES OF THE FRESH-WATER COPEPODA. 
I. Calanidw. Body elongate; abdomen more slender and quite 
distinct from the thorax. Antenne long, 23-to 25-jointed; in the 
male that of the right side modified for prehension. First four pairs 
of feet two-branched, outer branch three-jointed. Fifth feet several- 
jointed, those of the male prehensile; dissimilar on the two sides. 
Ovisac single. Habit strongly natatory. 
Il. Cyclopide. Cephalothorax compact, ovoid; abdomen slender. 
Antenne usually shorter than the cephalothorax, both geniculate in 
the male, 8- to 18-jointed. Second antennz (antennules) one-branched. 
Fifth feet rudimentary, a sixth pair sometimes present, especially in 
the male. Ovisacs double, lateral. Habit natatory. 
Ill. Harpacticide. Body cylindrical or flattened; abdomen not 
sharply distinct. Antenne short, not more than 10-jointed, those of 
the male both geniculate. Antennules palpate. First pair of swim- 
ming feet usually prehensile. Fifth feet foliaceous, somewhat dissimi- 
lar between the sexes. Ovisac usually single. Habit creeping or 
sub-natatory. 
FAMILY CALANIDE. 
This group is pre-eminently marine and contains diverse and 
graceful forms, mostly with very elongated bodies and antenne. Of 
the six genera here enumerated as more or less habituated to the use 
of fresh water, two are found as yet only in America and one is con- 
fined to Europe. 
