308 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
Length 0.46 mm. to 0.52 mm. Height 0.35 mm. to 0.36 mm. 
Width 0.26 mm. 
Shell inequivalve, one valve overlapping the other infront. Valves 
glossy, finely pubescent. 
In 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, asmall 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 sub-reniform, 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. AJll 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 wideas dorsal. 
Antennules typical for the genus. 
Antenne 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 and at its disto-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 sete, these sete 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 
sete. 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 
