SPECIES.
1. A. Cataracta. Shell thin, fragile, translucent, oblong oval, convex, cover. ed with a green olive, radiated, epider mis, within perlaceous ; beaks nearer central, frontal margin brown. ' Length, two inches and two-fifths : breadth, four inches and an half: conca vity of one valve nearly seven-eighths of an inch.
Plate 3. fig. 4.
This large muscle occurs in lakes, mill-. dams, &c. and bears some resemblance to the A. Anatinus of Europe. i Found by Mr. I. Lukens, in the deep part of a mill-dam. • 2. A. .11arginate. Shell very thin, fra gile, somewhat compressed, translucent, subovate ; epidermis green olive, paler on the disk and greener before ; anterior margin fuscous ; beaks nearer the poste rior end ; within bluish white, edged with whitish.
Length, one inch and a half: breadth, two inches.
Plate 3. fig. 3.
Resembles the preceding, but is more ovate, and the beaks are placed much further back ; it is very common in our rivers.
3. A. Undulate. Shell thin, fragile, convex, olivaceous, obscurely radiate and obtusely wrinkled ; umbo prominent, de corticated, with four or five obtuse un dulations disappearing on the disk. In the right valve, immediately under the beak, the margin is curved inwards for the reception of a corresponding margi nal projection of the opposite valve.
Length, nearly half an inch : breadth, nearly seven-tenths of an inch.
Plate 3. fig. 6• This species is perhaps rare : it does not exactly agree in all its characters with the genus, but approaches nearer it than to any other ; it resembles Alas modonta Undulate, for the young of which it might readily be mistaken.