l82 THE AMERICAN MIDLAND NATURALIST 



siliferous - 20 



A. — Striatula Zone. 



Calcareous shales and shaley limestones, slightly to very gritty. Weathers 

 partly to a rather gritty clay, partly to chips and nodules, strongly 

 iron stained. Beds indurated at various localities, as at Mason City; 

 at Rockford and Hackberry Grove, bearing large numbers of calcareous 

 concretions in the lower portions. Fossiliferous, the fossils occurring 



largely as casts. 25 



Disconformity. 



At Rockford the Striatula zone attains a total thickness of 

 above sixteen feet, and is separated into three quite distinct divi- 

 sions. The lowest of these, about two feet in thickness, does not 

 depend so much upon its fauna as the distinctive feature of a 

 large number of calcareous concretions, containing considerabla 

 amounts of pyrite, and occasional crystals of feldspar. Fossils 

 are conspicuous by their general absence, even poorly preserved 

 casts being uncommon. 



The second division, (Whitney i Faunule, Fig. 3), contains the 

 typical Striatula fauna. The less common species of this faunule 

 were described by Webster in a paper in the American Naturalist'; 

 the most common ones are: Schizophoria striatula (Schloth.), 

 A. reticularis (Linn.), A. hystrix Hall, and Spirifer whitneyi Hall. 

 The total thickness of the faunule is about ten feet. 



The third division, (Gypidula faunule. Fig. 3), is, to a considerable 

 extent, a transition between the Striatula and the Spirifer zones. 

 The fauna has lost its typical Striatula aspect, tut yet it is suffi- 

 ciently distinct from that of the Spirifer zone to be placed with 

 the lower division. Among the typical Spirifer zone species appear- 

 ing in this faunule are those marked by an asterisk in the faunal 

 list. The most interesting of these is a large, undescribed species 

 of Gypidula, commonly called G. comis (Owen). This form is, in 

 general, rare throughout the formation, but in limited areas of 

 the Gypidula faunule considerable numbers of specimens, usually 

 badly crushed, may be found. I collected a total of thirteen speci- 

 mens, only two of them good, from an area that could be covered 

 1)y an ordinary sheet of paper. In the area having a radius of 

 approximately fifteen feet about this find I have collected but four 

 specimens, all fragmentary. 



The accompanying diagram will serve to show the division of 

 the Striatula into faunules at the localities studied. The division, 



'Am. Nat. XXII., pp. 1013-1018. 



