DISTRIBUTION AND OCCURRENCE OF EURYPTERIDS 523 



to these pools and absent in the corresponding deposits like the Bosendale, 

 which are more open to the sea. Again, the fauna is a limited one, with 

 typical marine organisms only sporadically represented. How is it pos- 

 sible that marine organisms, which could migrate only along a coastline, 

 should be so distinct in adjoining pools with similar bionomic conditions? 

 Moreover, if these lagoons were peopled from the sea, how it is that there 

 is not an accompanying typical marine fauna, such as always inhabits 

 lagoons and estuaries? The existence of these pools in such close prox- 

 imity to each other, with distinct faunas, seems to me to be fatal to the 

 hypothesis of a marine habitat of these Merostomes. Per contra, how- 

 ever, this separation is precisely what we should look for if the organ- 

 isms were river forms. We have an excellent illustration of this in the 

 river trout of western United States. The Columbia and Missouri rivers 

 interlace at their headwaters, and here we find the cut-throat trout, Sal mo 

 darki. Away from this headwater region the species become gradually 

 differentiated. Thus the nearest relatives of 8. darki are 8. virginalis, 

 in the basin of Utah, and 8. stomias of the Platte River. ''Next to the 

 latter is Salmo spilurus of the Eio Grande, and then Salmo plewiticus 

 of the Colorado. The latter in turn may be the parent of the Twin Lakes 

 trout Salmo macdonaldi. Always the form next away from the parent 

 stock is onward in space across the barrier." 120 It is easy to see that 

 river organisms thus gradually differentiating as they separate along the 

 different waters under the influence of isolation might be swept into ad- 

 joining basins from two distinct rivers and so be buried as distinct 

 faunas, yet having relationship through a common ancestor. We can 

 readily understand that the Eurypterus fischeri of the Baltic provinces 

 might have its near relative in E. remipes of the Herkimer region, while 

 this in turn has a close relative in E. lacustris of the more western Buf- 

 falo region if the river systems in which these species lived interlaced at 

 their headwaters. Such relationship could be understood also for organ- 

 isms migrating along the coast, but such migration could hardly be ac- 

 complished by organisms primarily at home in lagoons which are unfa- 

 vorable to other marine life. 



The preservation of these Eurypterids in the water-limes reminds us 

 forcibly of the marvelous preservation of the land and marine organisms 

 in the Solnhofen beds of Bavaria. Walthcr 121 has shown thai these or- 

 ganisms did not live in the lagoons in which the fine deposits o( lime 



120 D. S. Jordan: The origin of species through Isolation. Science, n. b., Nov. :?. 1905, 

 p. 647. 

 i» Johannes Walther: The Solnhofen Plattenkalke blonomlscn betrachted. Festschrift 

 . zum 70ten Geburtstage Baeckel's. 



