152 UNIONID&. 
tightly on it, when it can easily be drawn out of the mud. 
If the stick be not flattened the closing force of the ani- 
mal will often break the shell. 
Var. I. avendria (Schroter). Broader, the upper and 
lower margins nearly parallel, the inferior more or less 
straight. 
Var. II. ~vostrdta (Rossm.). Ovate, upper margin 
forming a crest. 
Var. III. stagnélis (Sow.). Thin, inflated, antenor 
end: rounded; hinge line arcuate; beak sub-acute; 
basal line much rounded ; olive green. 
Var. IV. cucrasséta (Shepp.). More swollen and solid. 
Var. V. vradidta (Mill). Larger, yellowish-green, 
streaked ; beaks placed at a distance of only one-third 
from the anterior side. 
Var. VI. pédliida ( Jef.). Light yellow, wedge shaped. 
2. A. ANATINA ( [eaten by] ducks) Linné. Pl. VIIL., f. 2. 
Oval, compressed, glossy, green or brown; /¢gament short, promi- 
nent, straight, forming an angle with the lower margin; sca7s deeper 
than those of the last species. L. 2in. B. 3°5 in. 
Found in Britain as far north as the Forth, but not in 
Ireland. It may be distinguished from the last species by 
its smaller size and the angle which its hinge makes with 
the lower margin. It is considered a variety of A. cygnea 
by some, and by others as the young of that species. 
