THE CHESTER SERIES IN ILLINOIS 291 



the Renault-Shetlerville rests unconformably upon the Ste. Gene- 

 vieve Hmestone, and this unconformity must represent a time 

 interval not only equivalent to that between the Renault and Aux 

 Vases in Monroe and Randolph counties, but a very much greater 

 time during which the Aux Vases sandstone was deposited and also 

 the time interval preceding the Aux Vases during which the under- 

 lying Ste. Genevieve and St. Louis Hmestones were solidified and 

 their secondary chert formed, following which the whole of the 

 Ste. Genevieve and a part of the St. Louis limestones were removed 

 by erosion in some parts of the region. The unconformity repre- 

 sented by all of these events in Mississippian history must be con- 

 sidered as being of great importance in the classification of the 

 Mississippian as a whole. 



The limestones of the Renault are all more or less fossiliferous 

 wherever they occur, and in some localities faunas of considerable 

 magnitude can be secured. One of the forms which can be found 

 with careful search, wherever good exposures of the Renault are 

 present, is the crinoid Talarocrimus. This crinoid genus is repre- 

 sented by several species whose geographic distribution is some- 

 what different, but the same species is known to occur in localities 

 as far apart as Monroe and Hardin counties. A peculiar feature 

 of the genus is its two basal plates, and nearly all of the Renault 

 species have the suture between the two plates somewhat impressed, 

 giving to the base a distinctly bilobed form. These bases and the 

 separated radial plates are the portions most commonly met with, 

 and from these fragments the species cannot be certainly deter- 

 mined, but these bases alone seem to be sufficiently characteristic 

 to be distinctive of the Lower Chester faunas, and they are much 

 more commonly met with in the Renault than in the Paint Creek, 

 the higher limestone unit of the Lower Chester. Another fossil 

 form which is very characteristic of the Lower Chester beds, is the 

 bryozoan Cystodictya lahiosa, which occurs in both the Renault 

 and the Paint Creek, but has nowhere been observed in any higher 

 formation. The Renault fauna can be differentiated from that of 

 the higher Paint Creek Hmestone, among other ways, by reason of 

 the much less number of Archimedes and Pentremites, representa- 

 tives of both of these genera being very conspicuous in the Paint 



