EAELY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 



25 



Ptychopyge angustifrons var. gladiifera Schmidt, B3a 

 Ptychopygecincta Brogger, B2c. 

 Ptychopyge glohifrons Eichwald, B3c. 

 Ptychopyge inostranzewi Lamansky, Bib. 

 Ptychopyge knyrkoi Schmidt, B2c. 

 Ptychopyge limhata AngeUn, B2c, B3a. 

 Ptychopyge pahleni Schmidt, B2a, B2b, B2c. 

 Ptychopyge plautini Schmidt, B2b, B2c. 

 Ptychopyge truncata Nieszkowski, B2c. 

 Ptychopyge truncata var. brbggeri Schmidt, B2c, 

 B3a. 



Remopleuriies elongatus var. elongata Schmidt, C2. 

 Bemopleurides emarginatus Tomquist, Fl. 

 Remopleurides nanus Leuchtenberg, B2a, B3a, B3b 



B3c, Cla. 

 Sphxrexochus angustifrons Angelin, C3, Fl. 

 Sphserexochus conformis Angelin, Fl, F2. 

 Sphxrexochus roemeri Schmidt, Fl, F2. 

 Sphxrocoryphe cranium Kutorga, Clb, C2. 

 Sphxrocoryphe hubneri Schmidt, C3. 

 Sphxrocoryphe granulata Angelin, Fl, F2. 

 Trinucleus seticornis Hisinger, Fl. 



ORDOVICIAN FORMATIONS OF UPPER MISSISSIPPI VALLEY. 



The general time scale of early Paleozoic deposits in North America 

 is indicated in the table on page 38, but of all these formations only 

 those of the Ordovician and early Silurian need be discussed in the 

 present work, since the Cambrian, Ozarkian, and Canadian contain, 

 with two possible exceptions, no Bryozoa. 



In the preceding pages I have pointed out that the bryozoan faunas 

 of Baltic Russia from formations hitherto assigned to the Ordovician 

 (Lower Silurian), are closely allied to those contained in strata of 

 Middle Ordovician and early Silurian age in northern North America, 

 and especially in the upper part of the Mississippi Valley. The strati- 

 graphy and paleontology of these two time divisions have received 

 much study in Iowa, and more especially in Minnesota, so that the 

 section in these two States can now be used for comparison. The 

 composite section and a discussion of the individual beds follows: 



Composite geologic section of Ordovician, northeastern Iowa and southeastern Minnesota. 



Feet. 



Early Silurian (Richmond group) — 



Ordovician: 



Trenton group — 



Stewartville dolomite — 



(6) Porous, soft, yellowish dolomite and mottled magnesian lime- 

 stone (Maclurea bed) containing the Maclurina manitobensis 



fauna 0-100 



(a) Unfossiliferous, sandy mudstone 0-20 



Prosser limestone — 



(c) Cherty, nonmagnesian, light blue, fine-grained limestone 10-20 



(6) Thin bedded, occasionally shaly, fine-grained, bluish lime- 

 stone containing the Fusispira bed fauna. I^ocally the 

 basal strata of this division contain the fauna of the Nema- 



topora bed 20-35 



(a) Thin bedded, shaly limestone containing large examples of 

 Receptaculites oweni at the top and the Clitambonites bed 



fauna at the base - 20-45 



Black River group — 

 Decorah shale — 



(J) Shaly gray limestone filled with a variety of Prasopora insu- 



laris 0-8 



