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[Illustration: DUCK POND, CORNER OF FOURTH AND MARKET STREETS (ABOUT 1700)]
THE DIATOMACEÆ OF PHILADELPHIA AND VICINITY
_ILLUSTRATED WITH SEVEN HUNDRED DRAWINGS BY THE AUTHOR_
The present contribution to the local flora is intended as an introduction to more extended research.
The study is of advantage in relation to the life history of aquatic animals, the determination of ocean currents, as proved by polar discoveries, the investigation of geological strata where other fossil forms are absent, and the analysis of water supply; and, when we consider the universal distribution of diatomaceæ in the earth, the water and even in the air and the enormous deposits formed in past ages and still forming, we are able to realize the importance of a knowledge of these complicated forms and their function of purification.
The absence of descriptive works of reference in available form in this country, the polyglot confusion of authorities abroad and the amount of time, patience and skill required in obtaining, preparing and examining specimens, render the study one of difficulty.
The bibliography is omitted, as it is understood by those who possess the works of reference, and but few synonyms are given, having but little, except historical, value, especially when it is considered that modern investigators have no access to many of the earlier collections, when any of these exist.
So far as the marine forms are concerned, it is probable that nearly all occurring north of Florida are here included, and the fresh-water species described represent a large proportion of those found east of the Alleghanies. All of the figures are drawn to the same scale, a magnification of eight hundred diameters, from specimens in my possession, nearly all of which were found in or near Philadelphia.