1 86 THE AMERICAN MIDLAND NATURALIST 



The difference of the fauna at one point from that at another is 

 far more easily seen in the results of a collecting trip than in a 

 collection or in faunal lists, for the difference is primarily one of 

 numbers of specimens, secondly of variations, and probably last, 

 of species. This last statement cannot, of course, apply to such 

 forms as Lioclema occidens, which, while abundant about Rockford 

 is uncommon (relatively, at least) at Bird Hill, and rare at Hack- 

 berry. 



The Owen Substage. 



II. D. — ^The Idiostroma Zone. 



The highest portion of the Hackberry Stage which is in place 

 at Hackberry Grove is composed of four to six and one-half feet 

 of rather heavily bedded limestone, gray, gray-buff, or buff in 

 color, and crowded throughout by two species of Stromatoporoids 

 commonly referred to the genus Idiosiroma. Associated with the 

 species are several species of Siromatoporella, and numerous large 

 specimens of Naiicopsis gigantea H. & W. At a small exposure 

 south and east of Hackberry Grove the lower portions of this 

 zone contain numerous corals, among them being Aulopora an- 

 nectens Clarke. Westernia crassa Webster, and W. gigantea Web- 

 ster have been taken from this zone at Hackberry Grove. The 

 limestones of the Idiostroma zone are very distinct, both litho- 

 logically and faunally from the underlying Spirifer beds. At all 

 points where they were examined they were characterized by the 

 two slender Idiostromas. 



II. E. — The Naticopsis Zone. 



The type locality for the Hackberry vStage is the Hackberry 

 Grove exposure, and this same exposure furnishes the typical 

 development of the Cerro Gordo Stage. The type locality for the 

 Owen substage, and its upper two zones is the Owen Grove quarry, 

 near Owen Creek, in Portland Township. At that locality there 

 are some thirty feet of dark buff or brownish limestone, shales, 

 shaley limestones, and dolomites overlying the Idiostroma beds, 

 and characterized by their abundant gastropod fauna, in which 

 the species Naticopsis gigantea predominates. The three species 

 of Westernia, two or three unidentified species of Ceneostoma, 

 and Floydia concentrica Webster, are among the other gastropods. 



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