CORRELATION 311 



correlate the lower and middle portions of the Mayville beds and their 

 equivalents in Wisconsin with the Edgewood formation of Illinois and 

 Missouri. The upper portion of the Mayville beds in Wisconsin, con- 

 taining numerous shells of Virgiana barrandei var., can not be so defi- 

 nitely correlated with any horizon of Alexandrian rocks in northeastern 

 Illinois. The stratigraphic position of the zone of Virgiana barrandei 

 var. in the upper Mayville beds, overlying the Edgewood portion of the 

 Mayville in apparent conformity, is similar to that of the Platymerella 

 manniensis horizon in northeast Illinois, which there immediately suc- 

 ceeds the Edgewood limestone without any distinct sedimentary hiatus. 

 However, in some places, but not everywhere, in southwestern Illinois 

 and eastern Missouri the Platymerella manniensis zone is separated from 

 the underlying Edgewood limestone by an erosional unconformity. 



The genera Virgiana and Platymerella are very closely related. The 

 genus Virgiana was defined by Twenhoferin 1914 to include the species 

 described by Billings as Pentamerus barrandei, later referred by Schu- 

 chert to' the genus Clorinda, and two of its varieties, all of which were 

 known only from the Anticosti region. Of this genus Twenhofel says : 4 



"In the Becsie River formation of the Anticosti section occurs the shell de- 

 scribed by Billings as Pentamerus barrandei, which in its young stages has all 

 the characters of a true Clorinda. With maturity, however, the shell attains 

 large size, becomes decidedly elongate, narrow, and pronouncedly galeatiform 

 and the fold and sinus become reversed, the latter being obliterated and trans- 

 formed into a fold by the development of an axial rib, and the former disap- 

 pearing through bifurcation of the initial fold producing a sinus at the mar- 

 gin. The interior is that of Clorinda." 



Some of the shells of Platymerella, defined by Foerste in 1909, show 

 no distinct mesial fold or sinus on either valve, but many of them have 

 a sinus in the dorsal valve and a fold on the ventral, the latter having 

 been formed by the repeated division of a single plication which occupied 

 the bottom of what was originally a sinus, as shown in plate 16, figures 11 

 and 12. The sinus in the dorsal valve of Platymerella was likewise de- 

 veloped by the repeated division of a medial plication or fold. How- 

 ever, the ventral valve of Platymerella is never so galeate, nor is the 

 mesial fold so keeled, as in the shells of Virgiana. It seems probable that 

 the Upper Mayville beds in Wisconsin belong to a different geological 

 province from that in which the limestone in Illinois containing Platy- 

 merella manniensis was laid down, and the direct correlation of these 

 horizons is not yet certainly established. 



* W. H. Twenhofel : The Anticosti Island faunas. Canada Geol. Survey, Mus. Bull, 

 no. 3, Geol. series no. 19, Oct., 1914, p. 27. 



