The present communication contains brief notices of some of the rarer and hitherto undescribed species of Diatomaceæ of the United States sea board, which have fallen under my observation during the last three years, together with a list of a few of the more characteristic and generally distributed coast species.

The forms to be described are mostly salt-water or brackish. A few species, however, known as fresh-water will be noticed where these have been found domesticated along with the marine.

I have endeavoured, as far as possible, to avoid describing species unless from perfect specimens; carefully rejecting all doubtful and imperfect forms. I have also endeavoured to guard against sources of error arising from the great variation in size, outline, and striation, and from the absence of certain and positive indications whereby the sporangial may be detected and classified with its typical variety. The want, however, of several important consulting...

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