there alive or dead, and their hard parts will be preserved with those of the typical marine species.
2. The number of genera and species in fresh water is as a rule very much smaller than that in the neighboring sea, although the number of individuals may be nearly as large. Entire classes of organisms are wanting, while other classes are represented by only a few genera and species. Fresh water organisms are distributed by rivers and are found living in lakes and lagoons on the subaërial portions of deltas, in lakes, playas and more rarely epicontinental seas, all of which water bodies are geologically short lived.
3. Brackish waters may be considered under two types:
(a) The brackish waters or land-locked epicontinental seas such as the Baltic show a range in salinity from that of normal sea water to that of the rivers which empty into such water bodies. The fauna is made up of a modified marine and a modified fresh-water fauna, and always has its nearest relatives in the open sea, on the one hand, and in the rivers and connecting fresh-water lakes, on the other. Since the species contributed by the marine waters are so much more numerous than those contributed by the rivers, the greater number of the species in the brackish water will be related to marine forms.
(b) The brackish waters of estuaries are not stable enough to have a fauna which may be considered endemic. The marine fauna lives along shore and in the waters not too much disturbed by the tidal current, but the organic remains found in the estuaries are marine. Moreover, along the coasts affected by the deposition of the tidal muds, no organisms live and it is only many miles away from the estuary proper that the marine forms are found whose hard parts, carried by the tidal currents and in time comminuted, finally come to settle on the floors of the estuaries and on the river banks. These hard parts are carried up the estuaries as far as the tidal current is felt and it is only above this point that the fresh water forms are found. In the very small area between the purely fresh and the dominantly marine waters is the brackish-water area in which there may be a small mixed fauna.
APPLICATION TO THE PAST
Armed now with a considerable wealth of facts drawn from the present, we may turn to the past in an attempt to set forth any available criteria which may be used in determining the nature of a given habitat.
