308 GEOL. AND NAT. HIST. SUllVEY OF MINNESOTA. 



Length 0.46 mm. to 0.52 mm. Height 0.35 mm. to 0.36 mm. 

 Width 0.26 mm. 



Shell inequi valve, one valve overlapping the other in front. Valves 

 glossy, finely pubescent. 



lu fresh specimens, near the cephalic end of the shell, there is an 

 irregular cross-shaped dorsal dark spot (Figs. 1, 2), the arms of which 

 extend latero-ventrad along each valve for about one half the height 

 of the valve. Near the caudal extremity of the shell there is another 

 irregular dorsal dark spot. This spot is somewhat fan-shaped, with 

 the expanded portion directed caudad. Occasionally, on the ventral 

 surface, about one-third the length of the valve from the cephalic ex- 

 tremity, a small dark spot is seen. In fresh specimens these markings 

 are very pronounced; but in alcoholic specimens, they are usually 

 more or less obscured. 



Viewed from the side (Fig. 1) the shell is subreniform, higher near 

 the middle. It is about twice as long as high. Dorsal margin, feebly 

 convex; cephalic margin, convex; ventral margin, nearly straight, 

 slightly concave in the middle; caudal margin convex. 



Viewed from above (Fig. 2) the shell is sub-elliptical, inequivalve, 

 one valve overlapping the other in front. Shell widest near the mid- 

 dle. Cephalic extremity about as wide as the caudal. All margins 

 convex. Hinge-line straight. 



Viewed from below (Fig. 3) shell presents the same outline as when 

 viewed from above, but the line of contact of the two valves is 

 sinuated. 



Viewed from the end, the shell is oval, widest in the middle, ventral 

 margin about as wide as dorsal. 



Antennules typical for the genus. 



Antennae of female (Fig. 4) composed of five joints, the joints 

 diminishing in size from the second to the most distal joint. The 

 second joint bearing at about the middle of its ental margin a long 

 seta. The next joint bears at about the middle of its ental margin the 

 usual biarticular sensory hair andatitsdisto-ental angle a moderately 

 long plumose seta, this seta extends to about the base of the terminal 

 claws. From the base of the penultimate joint arise five very long 

 non-plumose setse, these setae extending far beyond the tips of the 

 terminal claws. At the middle of its ental border this joint bears a 

 short seta, and opposite this on the ectal border there is a slightly 

 longer seta. From the disto-ental angle of this joint arise two short 

 setse. From the ectal half of its distal margin arise two long claws; 

 one of these claws extends beyond the tip of the claws springing from 

 the terminal joint, while the other extends nearly to the tip of those 

 claws. At the extremity of the terminal joint there are two large 



