HACKBERRY STAGE OF THE UPPER DEVONIAN. 1 85 



Fig. 4. — Faunules of the Spirifer Zone at Hackberry Grove. 



two localities than I at first supposed.' It is certain that the 

 horizon of dominant gastropod development at Rockford is above 

 that of dominant brachiopod development. The upper portions 

 of the zone are not present, so the development of the Stromato- 

 porella fauntile, if it was developed, cannot be determined. 



Three miles west of Rockford, at a roadside exposure known 

 locally as Bird Hill, the Spirifer consists of two distinct faunal 

 divisions. The lower is made up of compact clay-shales crowded 

 with small fucoids, but without many other fossils; the second is 

 of soft clay-shale similar to that of the Rockford pits, but with a 

 distinctly different fauna. Ltoclema occidens (H. & W.), common 

 at the Rockford pits, is here almost uncommon, but the smaller 

 bryozoa are to be found in abundance. Rugose corals {Charac- 

 tophyllmn nanum (H. & W.), Zaphrentis solida H. & W., Chono- 

 phylluni clUpticum (H. &. W.) are abundant, and on the whole are 

 of a larger size than at Rockford. The general aspect of the brachi- 

 opod fauna is different from that at Rockford, though the species 

 are much the same. There is a notable tendency among the Spin'fcrs 

 of the hungerfordi and ivhitneyi groups to develop wide forms, as 

 5. whitneyi prodiicius Fenton. 



I think it has been made plain in these descriptions that there 

 are distinct faunal differences in the Spirifer Zone. These faunal 

 differences seem to me to be dependent on locality and local en- 

 vironment rather than upon time. Doubtless stratigraphical 

 position and individual variations are related, but I am inclined 

 to lay the major part of the responsibility to local environment. 



^Ara. Jour. Sci, XLVIII, p. 363. 



