20.
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-- This collection contains a considerable number of specimens of a shell agreeing with the short specific character given by Lamarck of the above; but as it has not been figured, I have referred to it with a mark of doubt.

The shells are rather solid, white, or white variegated with purple, with numerous concentric wrinkles, which are more distinct nearer the margin; the umbones, covered with a thin pale periostraca, nearly smooth and polished, with a small purple spot, the inside white, with the disk and posterior slope purple; the anterior and posterior slopes distinct, the lunule and escutcheon deeply and distinctly sulcated; length fourteen-tenths of an inch; height one inch.
3.

Mactra ovalina, Lam.Hist.5 477.
This shell is nearly of the same shape as the last, but the anterior slope is rounded and circumscribed, and the posterior only marked by a raised line in the periostraca.

The shell is thin, white; with a pale brown and deeply grooved escutcheon.
4.