| Observations on the Unification of Geological Nomenclature. 291 
_ Platycrinus to the Kaskaskia; Calceocrinus and Melocrinus to the 
Warsaw; Codaster to the Burlington; Aptocystites, Ampheristocrinus, 
Callocystites, Coccocrinus, Crinocystites, Cystocrinus, Echinocystites, 
-Eucalyptocrinus, Glyptaster, Gomphocystites, Hemicosmites, Hetero- 
cystites, Holocystites, Lampterocrinus, Lyriocrinus, Macrostylocrinus, 
Myelodactylus, Pisocrinus, Saccocrinus, Spherocystites and Stephano- 
crinus are peculiar to it. 
Among the Bryozoa, Polypora extends to the Coal Measures; Cal- 
lopora to the Keokuk; Hemitrypa and Paleschara to the Lower 
Helderberg; and Hornera, Lichenalia, Sagenella, and Thamniscus are 
peculiar to it. 
Among the Brachiopoda, Spirifera passes to the Jurassic; Athyris 
to the Coal Measures; Centronella, Cyrtinu and Nucleospira to the 
Subcarboniferous; Coelospira to the Corniferous; Anastrophia and 
Rhynchospira to the Lower Helderberg; Monomerella and Trimerella 
to the,Guelph; and Cyrtia, Meristina and Skenidium are peculiar to it. 
Among the Gasteropoda, Platystoma extends to the Coal Measures ; 
Strophostylus to the Upper Helderberg; and Zremanotus is peculiar 
to it. | 
Among the Cephalopoda, Streptoceras is peculiar to it. 
Among the Lamellibranchiata, Goniophora extends to the Chemung; 
Megalamus to the Guelph; and Amphicelia and Palewocardia are 
peculiar to it. 
Among the Crustacea, Cyphaspis is peculiar to it. 
Of the 90 genera thus supposed to have commenced an existence in 
this group, 49 never passed beyond it, 8 became extinct in higher 
groups of the Upper Silurian, 13 became extinct in the Devonian, and 
20 passed into the Carboniferous or beyond. 
This group is remarkable for the abundance of its fossils, and for 
the appearance of such a large number of new genera, as well as for the 
exceedingly great proportion that became extinct within it. It is 
really the only universal group in the Upper Silurian, and, therefore, 
constitutes the great body of it, for the groups which succeed in this 
formation are quite local in distribution, in comparison. 
The Onondaga Salt Grovp.—This group is limited in its geographi- 
eal distribution, though it occurs in Pennsylvania, and extends from 
New York into Canada, and thence northwestwardly to Lake Huron, 
Mackinac, and the Lake Superior region. It was named from Onondaga 
county, New York, .and was defined by Vanuxem, in 1842, and further 
defined by Hall,in 1843. It consists, usually, of marls and thin shaly 
