444 



SILURIAN" STRATA OF UNITED STATES. [Ch. XXVIL 



Shells of the lowest known Fossiliferous Beds in Russia. 



Fig. 605. 



Siplionotreta unguiculata, Eichwald. 



From the lowest Silurian Sandstone, " Obolus 



grits," of Petersburg. 



a. Outside of perforated valve. 



b. Interior of same, showing the termination of 



the foramen within. 



Obolus Apollinis, Eichwald. 

 From the same locality. 

 Interior of the larger or ventral valve. 

 Exterior of the upper (dorsal) valve. 

 (Davidson.) 



Among the green grains of the sandy strata above mentioned, Pro- 

 fessor Ehrenberg has recently (1854) announced his discovery of remains 

 of foraminifera. These are casts of the cells ; and amongst five or six 

 forms, three are considered by him as referable to existing genera (e. g., 

 Textularia, Rotalia, and Gutlulina). 



SILURIAN STRATA OF THE UNITEn STATES. 



The position of some of these strata, where they are bent and highly 

 inclined in the Appalachian chain, or where they are nearly horizontal to 

 the west of that chain, is shown in the section, fig. 505, p. 388. But these 

 formations can be studied still more advantageously north of the same 

 line of section, in the States of New York, Ohio, and other regions north 

 and south of the great Canadian lakes. Here they are found, as in Russia, 

 nearly in horizontal position, and are more rich in well-preserved fossils 

 than in almost any spot in Europe. In the State of New York, where the 

 succession of the beds and their fossils have been most carefully worked 

 out by the Government Surveyors, the subdivisions given in the first 

 column of the annexed list have been adopted. 



Subdivisions of the Silurian Strata of New York. [Strata below the 

 Oriskany Sandstone, see Table, p. 426.) 



New York Names. 



1. Upper Pentamerus Limestone 



2. Encrinal Limestone 



3. Delthyris Shaly Limestone 



4. Pentamerus Limestone 



5. Tentaeulite Limestone 



6. Onondaga Salt-group 

 *7. Niagara Group 



8. Clinton Group 



9. Medina Sandstone 



10. Oneida Conglomerate 



11. Gray Sandstone 



12. Hudson River Group 



13. Utica Slate 



14. Trenton Limestone 



15. Black-River Limestone 



16. Bird's-Eye Limestone 



17. Chazy Limestone. 



18. Calciferous Sandstone 



19. Potsdam Sandstone 



British Equivalents. 



Upper Silurian (or Ludlow 

 Wenlock formations). 



and 



Middle Silurian (or Caradoc Sand- 

 stone). 



)■ Lower Silurian (or Llandeilo beds). 



Cambrian ? (or Lingula flags and 

 beds, older than " the Llandeilo.") 



In the second column of the same table I have added the supposed 

 British equivalents. All paleontologists, European and American, such 



