214 



THE AMERICAN MIDLAND NATURALIST 



geologic age and proper terminology for the shaley limestones and 

 clays surmounted by limestones that are here referred to as "Hack- 

 berry," I therefore consider the following correlation of terms 

 advisable. 



Zone Division Sub-stage Division 



Indicates Slight Distinction But No Division- 



This same formation has variously been considered as Hamilton, 

 Chemung, and Portage, and the forms described in the 23rd 

 Report of the New York State Cabinet were considered as Chemung. 

 To my mind, however, there can be no doubt that this formation is 

 a distinct stage. 1 



II. — Description of Varieties. 

 Strophonella reversa gravis nov. var. 



Shell semielliptieal, width about equal to or less than the length, 

 hinge line slightly or not extended beyond the width of the shell. 

 Ventral valve convex near the beak, flattened along the hinge line 

 and deeply concave below ; shell abruptly deflected near the margin. 

 Dorsal valve slightly concave near the beak, abruptly bent down- 

 wards below the center. Area common, that of the ventral valve 

 being usually the wider. Surface marked by strong and heavy 

 dichotomizing plications, simple near the beak, also frequently 

 bearing heavy lines of growth. Surface of shell punctate. 



Interior of ventral valve marked by quite deep muscular impres- 

 sions; vascular impressions large, flabelliform or double ovate, the 



1 Since sending this to the printer, Webster lias endorsed and accepted 

 the zone divisions and names that I note here. — c. 1.. P. 



