290 Cincinnati Society of Natural History. 
Of the twenty-seven genera thus enumerated as commencing in 
this group, five did not pass beyond it, seven became extinct in the 
Niagara, ten in the Devonian, and five passed up into the Carboniferous. 
Pentamerus oblongus, Spirifera radiata, Meristella cylindrica, 
Lingula lamellata, Caryocrinus ornatus, and many other species pass 
from the Clinton to the Niagara, so that the groups are strongly con- 
nected specifically, as well as graduating into each other. 
The Niagara Group.—This group was so named from its develop- 
ment at Niagara Falls. It was defined by Vanuxem, in 1842, and 
more fully by Hall, in 1848. It consists of limestones and shales, and 
sometimes sandstones, though changing its lithological characters and 
combinations in different and distant localities. It is one of the 
universal groups. 
In New York and the southern part of Canada, its thickness is from 
250 to 300 feet, on Grand Manitoulin Island, 560 feet; on Cockburn 
Island, 400 feet; in the Lake Superior region, 300 feet; in Ohio, Indi- 
ana, Illinois and Iowa, about 600 feet; and in Missouri about 200 feet. 
Its greatest thickness, however, is in Tennessee or Newfoundland, but 
the measurements that have been made at these places have left the 
exact thickness uncertain. 3 
It is so well characterized by its fossils, that there is, usually, no 
difficulty in determining it at any locality, nothwithstanding any 
change that may have taken place in its petrological characters. 
The distribution of the genera supposed to have commenced their 
career in this group, is as follows: 
Among the plants, Psilophyton extends to the Devonian, and 
Giyptodendron is peculiar to it. 
Among the Protista, Astrwospongia extends to the Hamilton; and 
Aulocopina and Paleomanon are peculiar to it. | 
Among the Polypi, Amplexus, Cyathaxonia, and Syringopora pass 
up into the Coal Measures; Striatopora to the Burlington, Antho- 
phyllum, Blothrophyllum, Cladopora, Diphyphyllum, Limaria, Lyellia, — 
Thecia, Thecostegites, and Vermipora pass up into the Devonian; 
Calceola to the Lower Helderberg; Baryphyllum to the Onondaga 
Salt Group; and Acanthograptus, Astreophyllum, Calophyllum, 
Calyptograptus, Cenites, Cystostylus, Dictyostroma, Ethmophyllum, 
Inocaulis, Plasmopora, Rhizograptus, Syringolites, Strephodes, and 
Vesicularia are peculiar to it. 
Among the Echinodermata, Cyathocrinus and Potervocrinus ex- 
tend to the Coal Measures; Actinocrinus to the St. Louis Group; 
