362 C. L. Fenton — The Hackberry Stage of 



works. In neither of these pits are there many of the 

 hard calcareous concretions that are so characteristic of 

 the lower portion of the. Stria tula beds at Rockford, and 

 which are also quite conmion at Hackberry. The amount 

 of iron seems very considerable; the shale oxidizes to a 

 deep yellow ochre color and pyrite is plentiful. 



In the lower portion of the Striatula beds at Mason 

 City there are few fossils other than the fucoids already 

 noted. I have found Schizophoria striatula and Atrypa 

 reticularis even in the lowest ledges and a little farther 

 up a few fragmentary casts of Spirifer whitneyi were 

 collected. As one nears the top of the beds, however, the 

 casts of A. reticularis, Schizophoria striatula and Spiri- 

 fer whitneyi become quite numerous, and forms akin to 

 S. whitneyi gradatus also occur. 



At Rockford the major part of the Striatula zone is 

 smooth-weathering, though rather gritty clay-shale, with 

 a harder zone of about two feet at the base that contains 

 many concretions and much pyrite. Just above this one 

 may find, in fair to good state of preservation, consid- 

 erable numbers of brachiopods, among them Leptostro- 

 phia canace (H. & W.), Douvillina arcuata (Hall), 

 Gypidula n. sp., Atrypa reticularis (Linn.) and Spirifer 

 whitneyi Hall. Farther up in the beds have been found 

 most of the other species noted in the list of the Striatula 

 zone (C, lib). 



B, II. The Spirifer Zone. 



The most striking, and possibly the most interesting, 

 of the divisions of the Hackberry is that one which Web- 

 ster and Fenton have designated as the Spirifer zone. 2 

 It is the third pronounced stratum at Hackberry Grove, 

 where it attains a thickness of about twenty feet. At 

 this point, as at most others, this bed is oxidized to a 

 light yellowish color, and near the top it contains bands 

 of light yellowish limestone with numerous selenite crys- 

 tals. The selenite is also present in considerable 

 amounts in the same beds at Rockford, in Floyd County. 



There are three quite distinct faunules of the Spirifer 

 zone at Hackberry Grove. The lowest is characterized 

 by large numbers of specimens of Naticopsis gigantea 

 H. & W., and the allied forms Floydia concentrica "Web- 



2 Descriptions of some New Brachiopods and Gastropods from the Devo- 

 nian of Iowa, by Carroll Lane Fenton. American Midland Naturalist, vol. 

 5, September, 1918. 



