THE SILURIAN PERIOD. 415 



precipitation might be increased, or the evaporation lessened, or both, 

 and as a result, the waters of the basin would become brackish and 

 even quite fresh, where joined by streams, and would thus permit 

 the introduction of a fresh- water fauna from the adjacent land waters. 

 If, at the same time, the limited and perhaps intermittent connection 

 with the sea still obtained, as it appears to have done during the Salina 

 stage, there would also be some opportunity for the introduction of 

 marine forms from the sea, especially at and near the points of such 

 connection. The variations in the ratio of precipitation to evapora- 

 tion might naturally lead to variations in the constitution of the 

 water of the basin, and hence to conditions favorable to variation in 

 the faunas, and to the mixing of those marine and fresh-water species 

 that could stand the greatest variation in salinity. This latter inter- 

 pretation is dynamically the simpler and seems to us the better to fit 

 the fossil evidence. 



; Eurypterids appeared prominently about this time in the faunas 

 of j certain beds in England, Wales, Scotland, Sweden, and Russia. In 

 these regions there is no association with salt deposits, a fact 

 wjiich seems to imply that high salinity, as such, was not an essential 

 condition. There, as in America, the eurypterids do not pass up into 

 such of the overlying beds as are well stocked with marine fossils, 

 although there is more or less association with certain marine forms. 

 On; the contrary, they seem to pass up into deposits of the " Old Red 

 Sandstone " type, whose other fossils are chiefly land plants and fishes. 

 In the Coal Measures of both continents similar eurypterids are found 

 fossilized with numerous delicate ferns, as though they had been buried 

 together in some quiet body of inland water. While these associa- 

 tions are not entirely decisive, they lend some notable strength to the 

 inference that these large crustaceans were fresh- or brackish-water 

 forms rather than true marine forms, and to the further inference that 

 the peculiarities of the fauna of which they are a conspicuous factor 

 were dependent upon the conditions of terrestrial or semi-terrestrial 

 waters, rather than upon those of true sea- waters. 



The appearance of scorpions. — It is at this time also that the earliest 

 known scorpions appeared both in America (New York) and Europe 

 (Scotland and Sweden). These were allied to the eurypterids, and 

 their association lends something of force perhaps to the preceding 

 considerations. The European forms have been thought to show 



