BUFFALO SOCIETY OF NATURAL SCIENCES 65 



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 cephalo thoracic appendages and the pres- 

 ence 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 un- 

 der 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. 1 



CLASSIFICATION OF RECENT AQUEOUS HABITATS 



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: (i) 

 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 "brack- 

 ish," 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, la- 

 goons, 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 



1 Some parts of the following classifications were presented by the writer at the 1014 meeting of 

 the Paleontological Society of America. 



