184 THE AMERICAN MIDLAND NATURALIST 



which oxidize to a light yellowish. There are but slight traces of 

 induration, and the oxidization is much more complete than in the 

 lower beds. Fossils are quite common as casts, Schizophoria stria- 

 tula (Schloth.), Atrypa reticularis (Linn.), A. hystrix Hall, and 

 Spirifer Whitneyi Hall being the principal species. This bed 

 composes the Reticularis faunule of Fig. 3. 



At the pits of the Western States Cement Company, some three- 

 fourths of a mile to the northwest of the Mason City pits the shales 

 below the Hackberry is being worked for the production of cement. 

 They are overlain by seven to nine feet of the Striatula Zone, 

 which is stripped from the smooth, plastic formation below. The 

 Striatula section, while incomplete in the extreme, nevertheless 

 shows a great difference from that at the Mason City pits. The 

 entire thickness of the indurated strata is represented by less than 

 six inches of hard, shaley limestone, bearing fucoids. Above this 

 are seven or eight feet of material very similar to that described 

 above as constituting the Reticularis faunule, but with very few 

 fossils, and those as very poor casts. Mr. A. P. Potts, of the Mason 

 City Brick and Tile Company tells me that the heavy, indurated 

 beds have virtually disappeared less than three-eighths of a mile 

 north of the Mason City pits. This is an extreme example of the 

 decided local variation that characterizes the upper Devonian of 

 this particular district. 



II. C. — The Spirifer Zone. 



The most striking, and certainly the most interesting, pale- 

 ontologically of the divisions of the Hackberry is the Spirifer Zone, 

 which at Hackberry Grove attains a thickness of approximately 

 twenty feet. An exact determination of the boundary between the 

 Spirifer and Striatula zones is made diihcult by the talus. 



This zone, as developed at Hackberry Grove, is divided into at 

 least three faunules, though their bounds have not been very care- 

 fully [worked out. The table on the following page gives these 

 faunules, with the distinguishing characters. 



The Striatula Zone in the neighborhood of Rockford, and par- 

 ticularly at the brick pits, is composed of much the same yellow, 

 calcareous shales, but with a much lesser tendency towards in- 

 duration than is to be noted at Hackberry Grove. The develop- 

 ment of faunules is more obscure than at Hackberry Grove, and 

 I am inclined to think that there is less parallelism between the 



