722 



FRESH-WATER BIOLOGY 



150 (138) Infero-posteal angle rounded, with small tooth or teeth. 



151 



151(166) Valves with longitudinal striae. . . . yl/awa Baird 1850 . . 152 

 General form subquadrate; compressed, not crested. Valve with supero-posteal angle 

 rounded or well marked; infero-posteal angle rounded, almost always without teeth. Fornices 

 broad; rostrum short and blunt, little exceeding the apex of the antennules. Antennules 

 short, thick; olfactory setae equal. Keel of labrum large, ordinarily rounded; the posterior 

 angle not acuminate. Feet, 5 pairs, rarely 6; the 6th, if present, rudimentary. Post-abdomen 

 broad, compressed, with various armature. Claws with i basal spine and denticulate. Color 

 yellow in some shade, varying from light to dark, with shade of brown in large species. All 

 species littoral. 



152 (153) Infero-posteal angle with 1-3 small teeth. 



Alona monacantha Sars 1901. 



In general form and appearance not unlike A . 

 intermedia. Valves distinctly striated; infero- 

 posteal angle rounded and with 1-3 small teeth. 

 Post-abdomen with g-io denticles; claws with 

 very long basal spine. Keel of labrum angled 

 behind. Length, 9 > o-35~0-4 nirn. 



This species may be confused with Alonella 

 karua (232). The length of the basal spine on 

 terminal claw offers a ready distinction. 



Louisiana; in weedy pools. 



Fig. 1 1 28. Alona monacantha. 



0.1 MM 



153 (152) Infero-posteal angle unarmed i54 



154 (155) Post-abdomen long, narrow; distal marginal denticles very long. 



See Oxyurella . . 140 



155 (154) Post-abdomen not notably long 156 



156 (157) Post-abdomen with marginal denticles only. 



Alona guttata Sars 1862. 



Form much hke A. costata, but usually smaller and dorsal margin less arched. Valves 

 smooth, striate, or tuberculate (var. tuberciilata Kurz). Post-abdomen short, broad, shghtly 

 tapering toward apex; truncate, angled, with longest marginal denticles at angle; denticles 

 8-10, pointed, small; no squamae. Claws with small basal spine. Post-abdomen of d without 

 spines; vas deferens opens behind claws, without any projection. Length, ?, ca. 0.4 mm.; 

 ^ , 0.3-0.35 mm. See Fig. 11 29, d. 



Not uncommon everywhere. 



Fig. 1 1 29. a, Alona quadrangularis; b, Oxyurella ienuicaudis; c, Alona costata; d, Alona guttata; e, Alona 

 rectangula; f, Alona rectangula var. pulchra; g, Alona intermedia. 

 These figures are not drawn to the same scale. 



