HACKBERRY STAGE OF THE UPPER DEVONIAN. 199 



prolific and as distinct a fauna, with as many forms known from 

 that formation alone, is by no means great. And when it is recol- 

 lected that the maximum composite thickness of the formation is 

 but one hundred feet, and the average thickness very much less, 

 there seems even less reason to call the formation a substage 

 Moreover, if one calls a formation a substage he implies that it is 

 a division of some stage. But since there is no known equivalent 

 of the Hackberry, to what stage might it be referred? 



v.— Summary. 



I. — The highest, and youngest, position in the Devonian of 

 Iowa is occupied by a formation whose known maximum composite 

 thickness is about one hundred feet. 



2. — This formation has been discussed under various formation 

 names. The name Hackberry, proposed in 1889, has priority over 

 other names and is more directly suitable to the formation than 

 any other name which has been applied to it. Other names should 

 therefore be regarded as synonyms, and should not be applied to 

 this formation. 



3. — The Hackberry formation is distinct, with no described 

 equivalent. It is correctly designated as a stage (or group, as that 

 term is commonly used). 



Explanation of Plate 

 Figs, i — 2. Airypa reiicularis a/to Fenton. A characteristic Spirifer zone 



form, found both at Hackberry Grove and Rockford. 

 Figs. 3 — 4. Spirifer whilneyi rockfordensis Fenton. An erratic form of 



considerable rarity, which I have found only at Rockford. 

 Figs. 5-^6. Spirijer whitneyi productus Fenton. A form of S. whitneyi 



seldom found at Hackberry, sparingly at Rockford, but characteristic 



of the Bird Hill exposure. Specimen from Rockford. 

 Figs. 7 — 10. Spirifer whitneyi gradatiis Fenton. Rockford, la. A form 



best developed in the Rockford district, in the middle portion of the 



Spirifer zone. 



Figs. 11-17. Spirifer cyrtinaformis helenae Fenton. Rockford, Iowa. 

 Specimens showing the slanting area and produced hinge-line which 

 is particularly characteristic of the Bird Hill specimens. Good examples 

 are less common at Rockford, and still less so at Hackberry. 



Figs. 18-22. Slrophonella reversa gravis Fenton. Rockford, lov/a. A form 

 which, although usually best preserved at Rockford, is more charac- 

 teristic of Hackberry Grove, Gigantea faunule. 



Figs. 23-25. Douvillina arcuata maxima Fenton. Hackberry Grove, Iowa. 

 A form* which is found mainly at Hackberry Grove. 



