1098 Wisconsin Academy of Sciences, Arts, and Letters. 
1897. Cyclops prasinus Forbes, p. 57, pi. XIX, figs.l and 2, pi. XX, figs. 
1 and 2. 
1898. “ fluviatilis Brewer, p. 135. 
1903. “ prasinus Graeter, p. 486, pi. 15, figs. 3 and 4. 
1909. “ fluviatilis Byrnes, p.'28, pi. XY, figs. 1 and 2. 
A small slender species. Its breadth exceeds one-half its 
length, and its length is rather more than twice the length of 
the abdomen exclnsive of the fnrcal rami. The first segment 
is two-thirds the length of the entire cephalothorax. The last 
segment has a row of very fine hairs. 
The first abdominal segment (PI. LXXIX, fig. 1) is very 
slightly enlarged at its anterior end and the whole abdomen- 
tapers gradually to the end of the furca. 
The furca is short, being about one and a half times the length 
of the last abdominal segment. The lateral seta is situated at 
about the middle. Of the terminal setae the outer and inner are 
short and slender, the inner being the longer. The second 
and third are produced, the third being much the longer. 
The first antennae (PL LXXIX, fig. 2) are composed of 
twelve segments and reach to the end of the third cephalothor¬ 
acic segment. The last three segments bear a very minute hya¬ 
line membrane. The antennae are bent in a way that is pe¬ 
culiar to this species. 
The spinous armature of the terminal segments of the 
exopodites of the swimming feet are 3, 4, 3, 3. 
The fifth foot (PL LXXIX, fig. 10) is composed of one seg¬ 
ment. It bears at the inner distal angle a slender spine, on 
the outer distal angle a seta, and between, on a conical base,, 
an elongated seta. 
The egg-sacs lie close to the abdomen. 
The females average about .48 mm. in length. This is con¬ 
siderably smaller than the sizes given for the European speci¬ 
mens. 
This species is widely distributed in Europe and has been 
reported from the Azores (Richard 1896), Ceylon (Daday ? 98) 
and Calcutta (Gurney 1906 and ,1907). It is known to occur 
in America from the Rocky Mountains to the Atlantic and as 
far south as Mexico. Its characteristic habitat is lakes rather 
