THE NATURAL HISTORY SURVEY. 165 



Cumberland basin was probably initiated in early Silurian time, 

 and continued to extend its boundaries while the waters were 

 being withdrawn from the Mississippian basin, perhaps to the 

 north, so that in age it is at its culmination younger than the 

 Silurian Mississippian sea. 



The age of the great mass of Silurian sediments of the 

 Cumberland basin is probably about equivalent to that of the 

 Salina beds of the interior, and is therefore younger than any of 

 the Silurian beds of the Chicago area. However, .these Cumber- 

 land basin faunas are the direct outgrowth from older Silurian 

 faunas, without doubt contemporaneous with the Niagaran of 

 the Chicago area and other portions of the interior, which flour- 

 ished along the eastern shores of the Atlantic basin, and at its 

 southern extremity in Maryland there are sediments which were 

 deposited contemporaneously with the Niagaran formations of 

 the interior. 



In the appended bibliographic list of North American Silur- 

 ian trilobites (pp. 185 to 210 of this Bulletin), one hundred and 

 five species are recorded, and of this number forty-one, or nearly 

 40%, are recognized in the Chicago area, this being the largest 

 number'of Silurian trilobites. recognized from any area of similar 

 size in America. 



With the exception of two or three figures, the accom- 

 panying illustrations have been made by Miss Mildred L. Marvin* 



