RECENT AQUATIC FAUNAS
Having established what seem to be fairly accurate limits for the ranges in salinity in all of the waters on the surface of the earth, it becomes possible to study the faunas of these different realms, for the type of life in any given water body is more dependent upon the salinity than upon any other physical factor with the exception of extremes of temperature. The absolute necessity of studying recent faunas with particular attention to the types of organisms represented, and to the numbers of species and of individuals, has not been realized sufficiently in the past. The habitats of fossil faunas cannot be determined without a knowledge, an intimate knowledge, of the habitats of recent faunas. To be sure, there is little doubt about the kinds of organisms which make up a typical marine fauna; in many cases, too, there may be no difficulty in recognizing a fresh water (especially lake) fauna, but there is an undoubted haziness and lack of precision in all ideas connected with brackish waters and with the faunas thereof. When a given fossil fauna has shown certain peculiar characteristics, such, for instance, as a complete or almost complete absence of molluscan representatives or when the fauna has been confined to one or two classes of organisms, the custom has been and still is to say that the organisms lived in brackish water. It is, thus, necessary to determine the nature of recent faunas which are characteristic of the various bionomic realms, in order that we may, not without a fair degree of certainty, establish the criteria for determining the faunal nature of the habitats of the past.
Marine. The marine fauna is always large and varied, comprising, typically, representatives from each taxonomic division among the invertebrates. Not only are there a large number of genera and species, but nearly all phyla are represented. For the mollusca alone the number of genera in a given region may run up into the hundreds and that of the species may be considerably over a thousand. The figures apply especially to the littoral zone, that belt along all coasts which is most favorable to life. There light penetrates to the bottom, the food supply is abundant, and varying substrata are available to suit the needs of different organisms. This zone, extending from high water approximately to the two hundred fathom line, is the one of greatest geologic interest because nearly all of the marine formations of the past were littoral; unequivocal abyssal deposits being very rare. Since practically all of the invertebrate organisms
