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48 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
requisite, and the limitations of range in themselves point to a more 
recent origin than the other groups of micro-crustacea; nor is it diffi- 
cult to trace the connecting links in many cases. 
KEY TO THE FRESH-WATER GENERA OF THE CALANIDE. 
I. FIFTH FEET OF THE FEMALE BI-RAMOSE. 
a, Inner rami of all the swimming feet three-jointed. 
* Inner ramus of the right male fifth foot three-jointed. 
+ Outer ramus of the right male fifth foot two-jointed. Limnocalanus, 48 
tt Outer ramus of the right male fifth foot three-jointed. 
Osphranticum, 85 
** Inner ramus of the right male fifth foot reduced. . . . *Boeckella. 
b. Inner rami of the first swimming feet two-jointed. . . . Diaptomus, 54 
c. Inner rami of all the swimming feet two-jointed. . . . . . Broteas, 80 
Il. FIFTH FEET OF THE FEMALE UNI-RAMOSE. 
a. All the swimming feet with the inner ramus one-jointed. 
* Abdomen symmetrical, not prehensile in the male. . Heterocope, 80 
** Abdomen unsymmetrical, with apse acne 5 in the 
Wales. a0 . . . . Epischura, 81 
b. First swimming foot with one;jointed inner ramus, the rest two- 
JOINLE. > eS cise a . . . . . . HEurytemora, 49 
c. Swimming feet with thine sound 4 inner rami. 
* Fifth foot of the male one-branched. . . . Pseudodiaptomus, 53 
** Fifth foot of the male with one-jointed innerramus. . . *Poppella. 
GENUS LIMNOCALANUS Sars. 1863. 
Cephalothorax six-jointed, slender, last segment never divaricate. 
Abdomen in the female three- or four-jointed (not including stylets), 
in the male five-jointed. Caudal stylets elongated with five sete. 
First antenne 25-jointed, shorter than the body; in the right antenna 
of the male a geniculate joint between segments 18 and 19. Second 
pair of maxille, strong, eight-jointed, last joint unguiculate. Feet 
bi-ramose, those of the first four pairs with three-jointed rami. Outer 
ramus of fifth feet in the female three-jointed, with a claw-like process 
from the second segment; inner ramus like those of the other feet. 
The outer ramus of the right foot of the male more conspicuously 
modified, apparently two-jointed, with a long apical claw. This 
genus very closely approaches Centropages, which latter is its marine 
prototype. The genus is represented by two species, one of which is 
circumpolar, the other occurring in China farther south. 
* Not described by the author. See plates. 
