478 
ELEMENTS OF GEOLOGY. 
Shells of the lowest known Fossiliferous Beds in Russia, 
Fig. 565. Fig. 566. • 
Sijjhonotreta unguiculata, Eichwald. From 
the Lowest Silurian Sandstone, “Obolus 
grits,” of St. Petersburg. 
a. Outside of perforated valve. 6. Interior 
of same, showing the termination of the 
foramen within. (Davidson.) 
Obolus Apollinis, Eichwald. From 
the same locality. 
a. Interior of the larger or ventral 
valve, b. Exterior of the upper 
(dorsal) valve. (Davidson, “Palse- 
ontograph. Monog.”) 
or Ungulite of Pander, and a Siphonotreta (Figs. 565, 566). 
Notwithstanding the antiquity of this Russian formation, it 
should be stated that both of these genera of brachiopods 
have been also found in the Upper Silurian of England, i, e. 
in the Wenlock limestone. 
Among the green grains of the sandy strata above men¬ 
tioned. Prof. Ehrenberg announced in 1854 his discovery of 
remains of foraminifera. These are casts of the cells; and 
among five or six forms three are considered by him as refera¬ 
ble to existing genera (e. g,^Textularia,Iiotalia^ and GuttuUna), 
Silurian Strata of the United States.—The Silurian forma¬ 
tions can be advantageously studied in the States of New 
York, Ohio, and other regions north and south of the great 
Canadian lakes. Here they are often found, as in Russia, 
nearly in horizontal position, and are more rich in well-pre¬ 
served 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 or base of the Devonian.') 
New York Names. ^ British Equivalents. 
1. Upper Pentaraerus Limestone . ~ 
2. Encrinal Limestone .... 
3. Delthyris Shaly Limestone . . 
4. Pentamerus and Tentaculite 
Limestones. 
5. Water Lime Group . . . . 
6. Onondaga Salt Group . . . 
7. Niagara Group. 
8. Clinton Group. . \ . . . 
9. Medina Sandstone .... 
10. Oneida Conglomerate . . 
11. Gray Sandstone. 
Upper Silurian (or Ludlow and Wen- 
lock Formations). 
^ Beds of Passage, Llandovery Group. 
