STRATIGBAPHICAL GEOLOGY. 7 



and appearance of the stones and marl to indicate tlie clianging character of the 

 fossil contents, to which I shall now call more particular attention. 



The Zygospira modesta is found throughout the group, varying in size from 

 a small pin-head to a pea. The largest size has been called Zygospira cincin- 

 natiensis. The smaller specimens differ in their proportional length and breadth 

 and in the proportional elevation of the middle of the ventral valve, and corres- 

 ponding depression of the mesial sinus of the dorsal valve. The larger speci- 

 mens differ in the same respect; and as the number of plications is but little in- 

 creased, they become larger and coarser. The same species from the Trenton 

 Group, at Ottawa, Canada, is more elongated, and more finely plicated than the 

 Cincinnati forms; while specimens from the Trenton Group of Southern Minne- 

 sota are scarcelj'' distinguishable from Cincinnati specimens of medium size. 

 This species passes through the Trenton, Utica and Hudson Eiver Groups, and 

 is found in the Clinton Group. 



Strophomena alternata is found throughout the Group. Specimens secured 

 within 200 feet of low water mark at Cincinnati, are large, thin, frail, and some- 

 what flat, but in their markings resemble the more profound specimens from the 

 Trenton Group of New York and Ottawa, Canada. Many specimens found from 

 350 to 450 feet above low water mark are peculiarly thick, firm and heavy. Fi-om 

 450 feet above low water mark to the top of the Group the specimens are, gener- 

 allj'^, proportionately longer on the hinge line and more distinctly eared than they 

 are below, and frequently much larger. One form of these long eared specimens 

 has been called variety loxorhytis. The variety nasuta is most distinctly 

 marked at an elevation from 400 to 450 feet above low water mark, where it is 

 thicker and deeper than the same variety from the Trenton Group of New York 

 and Canada. The variety alternistridta is most common in the middle and up- 

 per part of the Group. The variety /racia is fovmd only in a vertical range of a 

 few feet about the middle of the Group. This species is widely distributed and 

 ranges from the Chazy to the Clinton Groups, passing through a great many 

 forms, which, if constant or characteristic of particular geological horizons, would 

 be regarded as good species. 



Leptena sericea is found throughout the Group, changing at times in size, 

 length of the hinge line and comparative thickness. It is a common form in the 

 Trenton, Hudson Eiver and Clinton Groups. 



Strophomena tenuistriata is frail and rare in the lower part of the Group, 

 but quite common and well preserved in the upper part. This, including its 

 nearly related forms, under the names of rhomboidalis and dejyressa, is almost 

 world wide in its distribution, and ranges from the Trenton Group to the Lower 

 Carboniferous. One could not hesitate, however, in separating the Lower Silu- 

 rian from the Upper Silurian forms, and these again from the Devonian and 

 Lower Carboniferous forms, while remarking the somewhat general resemblance 

 between them. 



Streptorhynchus hallia is found in the lower 200 feet of the Group, and is 

 not known to occur elsewhere. 8. planoconvexa occupies only a few feet in ver- 

 tical range about the middle of the Group. S. nutans, S.planumbona, S.sub- 

 tenta and S.flUtexta are confined in their range to the upper part of the Group; 

 though S. subtenta is found in the Hudson River Group at English Head, Anti- 

 costi, and 8. filitexta in the Trenton Group of New York. 



