XCV1 THE PALEONTOLOGY OF MINNESOTA. 



The Black River shales of Minnesota (Trenton shales of this volume particularly 

 those parts which are distinguished as middle and upper thirds) may conveniently be 

 divided into four beds as follows : 



Rhinidictya bed. This bed is usually referred to in the following descriptions as the 

 " middle third of the Trenton shales." It consists of dark green, soft shale, not over 5 

 feet thick in Fillmore county, and between 20 and 25 feet in Goodhue and Ramsey counties. 

 (See sections 1, 2, 6, 7 and 8). It is very fossiliferous, particularly in the upper half, 

 where the fossils, Bryozoa mainly, occur in thin calcareous layers in great quantity and 

 variety. Rhinidictya mutabilis is exceedingly abundant, as is also Batostoma winchelli, while 

 no less than 25 of the 57 species of Bryozoa may be said to be common. Excepting the 

 Mollusca, which occur as casts of the interior, the fossils are in an excellent state of 

 preservation. 



Ctenodonta bed. At St. Paul this bed is scarcely distinguishable from the Rhinidictya 

 bed, several of the leading Bryozoa being quite abundant in it. However, in Goodhue 

 county, where the bed is from 6 to 10 feet thick, Bryozoa are almost entirely absent while 

 the Mollusca occur in great numbers. No less than 43 Lamellibranchiata have been found 

 here and over half of the number are restricted to this bed. The Gastropoda are almost 

 equally numerous while the Cephalopoda are, as far as number of species is concerned, 

 nearly as well represented here as in any other division of the Lower Silurian in Minne- 

 sota. The bed contains considerable iron and in Goodhue county can always be recognized 

 by the red or brown color of the fossils and weathered slabs. The latter frequently 

 become oolitic exteriorly, the grains being concretions of limonite of lenticular form. 

 Ctenodonta socialis, C. scofieldi, C. compressa, C. planodorsata, Lyrodesma acuminatum, 

 Matheria rugosa, Whitella scofieldi, Cyrtoceras comiculum, Archinacella deleta, Raphistoma 

 peracuta, Lophospira oweni, and L. spironema, are some of the characteristic fossils. 

 Under the descriptions of some of the Lamellibranchiata this bed is distinguished as the 

 "upper part of the middle third of the Trenton shales." The name Ctenodonta bed is 

 used only in the chapter on Gastropoda. 



Phylloporina bed. This bed has a thickness of from 10 to 15 feet in Ramsey and 

 Goodhue counties, but it is much thinner in Fillmore county. It much resembles the 

 Rhinidictya bed, and like it contains a great number of Bryozoa, but instead of the 

 bifoliate forms it is the monticuliporoids that predominate here. Homotrypa subramosa is 

 very abundant at most localities, as are also Prasopora simulatrix, P. conoidea and Batostoma 

 montuosum, but the most distinctive and easily recognized fossil is the Phylloporina corticosa. 

 Of four Echinodermata one, Agelacrinus rrtnrginatus, occurs in the "Upper Blue" in Wis- 

 consin, and at the base of the Trenton in Kentucky. Lamellibranchiata, Gastropoda, 

 Cephalopoda and Trilobita are rare, but the Ostracoda are well represented and some of 

 the species are abundant. In the descriptions of the fossils this bed is not separated from 

 the following, the designation "upper third of the Trenton shales " applying to either one 

 or both. 



