Review of the New York Geological Reports. 305 



limestone of the upper Silurian ; but it is most abundant, and 

 characterizes more especially the Wenlock and Dudley lime- 

 stones. In Iowa and Wisconsin it is a remarkably abundant fos- 

 sil in the upper magnesian Pentamerus limestone ; thus, so far as 

 this fact goes, it rather indicates that the strata containing them 

 are upper Protozoic. 



Furthermore, the Clinton group of New York has but little 

 lithological analogy with the Pentamerus and Catenipora lime- 

 stone of the northwest. The strata of the latter region are 

 magnesian limestones, remarkably free from shaly or agillaceous 

 intermixture ; the Pentamerus rock of New York is included be- 

 tween very thick beds of green shale. In Indiana and Ohio the 

 Pentamerus beds of the cliff formation, though they do not con- 

 tain as much magnesia as the equivalent rocks of the northwest, 

 are nevertheless magnesian without any intercalation of shale.* 



Below are other fossils found in the limestones of the Clinton 

 group : 



Hall's Report, p. 71. 



Pig. 1. Orthis Circulus. 2. Atrypa congesta. 3. Atrypa na- 

 viformis. 4. Atrypa plicala. 5. Crinoidal joint {Actinocrinus 

 plumosus ?) natural size and magnified. 



Of these the most widely distributed is fig. 5, being found 

 throughout the western extent of this formation in New York, 

 and also in Ohio. Fig. 2, says Hall, " appears to take the place 

 of the Pentamerus at some localities, so far as regards numbers 

 of individuals, prominence, and place in the strata." 



A beautiful little semicircular Strophomena occurs at Roches- 

 ter in the upper green shale, to which the name S. elegantula 



* The strata discovered by Dr. Locke, containing oolitic iron and some fossils 

 identical with those of the Clinton group of New York, lie lower than those con- 

 taining the Pentamerus ; they occupy the same geological position as the " flinty 

 limestone" of his Report, i. e. immediately above the blue limestone. 



