Page:The Habitat of the Eurypterida.djvu/75

 along all coasts even where no rivers enter as well as at considerable distances from shore beyond the debouchures.

Walther makes mention of the occurrence of great rafts of trees off the mouth of the Congo, 450 km. from shore, some of these interlocking tree islands being 100 m. across. Agassiz likewise has noted in the Caribbean Sea, Helix, leaves, and other land organisms dredged from a depth of 1000 to 2000 fathoms, which is far beyond the littoral zone.

Thus we must conclude that in the marine waters, and especially in the littoral zone, there is not only an abundance of invertebrate organisms of nearly all phyla, but there are stragglers from other realms; insects and plants are blown out to sea, while terrestrial animals and vegetal remains, together with fluviatile organisms, are carried along by the rivers, all at length being entombed in the marine sediments with the hard parts of the organisms which lived and died in the sea. In such cases we should expect to find the fluviatile and terrestrial remains shattered and worn on account of being transported oftentimes for a considerable distance, and usually subject to partial destruction by the débris which the rivers carry. At any rate, it is apparent that it is customary, not anomalous, for the remains of terrestrial and marine organisms to occur together.

. The fresh-water faunas of rivers and lakes, on the other hand, present quite different features. While the number of individuals in a given river or lake may indeed be large, the number of genera and species is very small as compared with those in the neighboring marine waters. Furthermore, there are only a few large classes abundantly represented, such as the fish, molluscs, and protozoa, while all of the other classes, so well represented in marine waters, are in given rivers or lakes represented often by a single species only, or by none at all. Three comparative sets of figures for the molluscs will serve to illustrate how small the number of genera and species is in fresh water when compared with those in marine waters at approximately the same latitude.