E. M. Kindle — Silurian Fauna in Western America. 129 



and well to the eastward of the McKenzie River in British 

 America, The conclusion reached by Weller is that the route 

 of migration between Europe and America was northeastward, 

 by way of Hudson Bay, northern Greenland, and Spitzbergen 

 to northern Europe. The presence of a Silurian fauna in 

 northern Alaska necessitates the consideration of an alterna- 

 tive hypothesis. The occurrence there of a Niagaran fauna 

 requires the shifting of Weller's hypothetical western bound- 

 ary of the Silurian sea at least a thousand miles to the west- 

 ward in the northern third of the continent. It gives much 

 weight to the northwestern route as the probable path followed 

 by Silurian faunas in traversing the area between Europe and 

 America. Between the Silurian faunas of northern Russia 

 and northeastern Alaska there intervene the nearly continu- 

 ous land masses of Siberia and western Alaska. That the 

 present shallow marine conditions of the continuous continen- 

 tal shelf were represented by somewhat similar conditions 

 across northeastern Asia during the Silurian is suggested by 

 the presence of Ordovician strata in western Alaska. 



In this connection the known distribution of the peculiar 

 brachiopod Streptis greyi deserves consideration. It occurs in 

 England, the island of Gotland, Bohemia, Alaska and Arkan- 

 sas, while it is entirely unknown east of the Mississippi River 

 and in northeastern America. This peculiarly developed spe- 

 cies forms an important connecting link between the faunas of 

 Arkansas, Alaska and Europe, and its apparent absence from 

 the eastern third of America points toward a northwestern, 

 rather than a northern or northeastern, connection between the 

 Silurian faunas of Europe and America. When the Siberian 

 faunas have been studied it is very probable that Silurian fau- 

 nas will be found in northern Asia connecting those of Alaska 

 with the Silurian fauna of northeastern Russia. 



IT. S. Geological Survey, Washington, D. C. 



