THE WATER FLEAS (CLADOCERA) 719 



130 (131) Dorsal margin much arched. . . . Acroperus harpae Baird 1835. 



Eye and ocellus near margin. Ros- 

 trum acute. Eleven to twelve groups of 

 fine spinules on post -abdomen. Length, 

 9 , to 0.8 mm.; $ , to 0.6 mm. 



Common everywhere, among weeds, 

 in relatively open water; not in muddy 

 pools. 



FIG. 1 121. Acroperus harpae. 



O.I MM. 



131 (130) Dorsal and ventral margins nearly straight. 



Acroperus angustatus Sars 1863. 



Crest larger than in A. harpae; eye and 

 ocellus removed from margin and rostrum 

 obtuse. Length, 9> to -9 mm.; $, 0.6 

 mm. 



Common in similar situations to preced- 

 ing species. Transition forms between these 

 species may be found and very probably 

 they should be united. 



FIG. 1122. Acroperus angustatus. 

 U.I MM. t ^ ' 



(124, 129) No crest 133 



J 33 ( J 34) Valves not tumid; post-abdomen broad. 



Alonopsis Sars 1862 



O.I MM. 



1330 



General form resembling Acroperus but less com- 

 pressed and without crest. Keel of labrum moderate 

 or small, almost triangular. Valves obliquely striated 

 but striae often inconspicuous. Post-abdomen long, 

 broad; with well-developed marginal denticles. Six 

 pairs of feet, the last very small. $ with usual 

 characters. Color yellow. 



FIG. 1123. a, Alonopsis elongate; b, Alonopsis aureola. 

 (After Doolittle.) 



15-17 marginal denticles ..... Alonopsis elongata Sars 1861. 



Minute tooth at infero-posteal angle of valves. Post-abdomen with lateral fascicles. Length, 

 9 > ca. 0.8 mm. 



( T 33 a ) About n marginal denticles. 



Alonopsis aureola Doolittle 1912. 



Both 



No lateral fascicles or infero-posteal tooth. Length, 9 ca. 1.9 mm.; $ unknown. 

 species in margins of lakes and ponds among weeds. Rare; reported only from Maine. 



