414 GEOLOGY. 



thus fossilized with marine forms. Their occasional presence in the 

 earlier periods is thus explained, while their seemingly sudden appear- 

 ance in abundance and in gigantic forms in the closing Silurian, and 

 their prominence in the land-water deposits of the Devonian and Car- 

 boniferous finds ready explanation in the fact that these are the first 

 well-preserved fossil-bearing deposits of land waters. 1 In these deposits 

 the eurypterids often appear without any marine associates, while 

 occasionally there are some marine or at least brackish-water forms 

 associated with them, implying either that they lived in brackish 

 or salt water at times, or that their remains were carried out into such 

 waters by the land streams or estuarine currents. 



These doubts as to the habitat of the eurypterids lend special inter- 

 est to their fine development in the Waterlime beds from which mol- 

 lusks, crinoids, corals, and similar marine forms are almost entirely 

 absent. The few brachiopods found are usually pauperitic, as though 

 they lived in uncongenial conditions. The occasional presence of a 

 few undoubted marine forms does not so much indicate that the waters 

 were habitually saline as merely that they were occasionally and 

 partially so. 



It is suggestive, in this connection, to note that these great crusta- 

 ceans, appearing thus abruptly in the Waterlime deposits, are not 

 found in the true marine beds that overlie the Waterlime, as might 

 be expected if they were true marine types recently come into the 

 seas, or into fossilizable form. It seems, therefore, rather more rea- 

 sonable to regard their normal habitat as connected with the special 

 conditions that prevailed during the Waterlime formation, than with 

 the typical conditions of the sea. The Waterlime formation is uni- 

 versally regarded as representing a stage of emergence from the exces- 

 sively salt Salina stage. Emergence from this stage might take either 

 of two forms — (1) The connection of the basin with the sea might 

 have become more ample, and the excessive saltiness of the water might 

 be reduced by interchanging currents to the normal salinity of the 

 sea. In this case the marine life should creep back into the region in 

 proportion to its endurance of briny water, and the fauna should always 

 be truly marine. (2) On the other hand, without any material change 

 in the attitude of the surface or in its connections with the sea, the 



1 On the Habitat of the Early Vertebrates, T. C. Chamberlin, Jour. Geol., Vol. VIII, 

 1900, pp. 400-412. 



