Page:The Habitat of the Eurypterida.djvu/71

 occurrences in mind. In considering the habitats, we may confine ourselves to those which are aqueous, since certain anatomical features, such as the nature of the cephalothoracic appendages and the presence of branchiae on the abdominal appendages, establish beyond a doubt the fact that the eurypterids lived in the water and not on the land, as do their near relatives the scorpions.

Before attempting to draw conclusions about the conditions under which the Eurypterida must have lived, it is necessary to have in mind the physical and faunal characteristics of the various types of habitats. Since these characteristics have never, so far as the writer knows, been discussed at length, I shall here state some of the results of an extended study of aqueous habitats which in time will be published as a separate paper.

The most natural and fundamental characteristic which can be recognized in classifying aquatic bionomic realms is salinity, on which basis it is readily seen that there are only two original habitats: (1) marine, (2) terrestrial fresh water. Animals living either in marine or in fresh waters may become adapted to water which is of a salinity intermediate between the other two and generally designated "brackish," or to a salinity greater than that of normal marine waters. Thus to the two original types of habitat may be added two others: (3) brackish water, and (4) super-saline water, which are never original habitats. By this is meant that, minor variations excepted, no aquatic forms ever originate in the brackish water of estuaries, lagoons, cut-off arms of the sea, or interior basins, or in the super-saline waters of lakes. (This will be demonstrated below, pp. 73, 76, 77.) That this should be the case is due to the evanescent character of such water bodies. It is, of course, conceivable that a body of this type, long persistent, might be peopled from the land or from another aqueous realm and that such a fauna might be specialized. To these principal types may be added certain minor ones, giving seven in all. In the following table are given the salinity types, and with them what seems to be the best salinity ranges. It is not of importance here to go into the reasons for the making of the limits, but it may be said that they are based on a large number of typical examples in each