292 Cincinnati Society of Natural History. 
limestones containing salt and gypsum. In Wayne county, New York, 
it has a thickness of 1,000 feet, but about Grand river, in Canada, it 
is reduced to 300 feet; about Mackinac, to 40 feet; and in the Lake 
Superior region, to less than 50 feet. 
It is not very fossiliferous, and only three genera are known to have 
commenced their existence in it, viz: Hurypterus which passed up as 
high as the Coal Measures, Pleurodictyum which occurs in the Lower 
Helderberg, and Zusarcus which is peculiar to it. It was called the 
‘Onondaga Formation,” by the Canadian Geologist, in 1863, and it is 
an improvement to drop the word “salt,” and call it simply the 
Onondaga Group. 
The Guelph Group.—The Niagara Group is succeeded in western 
Canada by a lenticular mass, having a maximum thickness of about 
160 feet, which was defined by Sir Wm. Logan, in 1863, as the Guelph 
Group, from the town of Guelph, where it is well exposed. It is a 
limestone dolomite, and may have been a brackish water deposit. It 
is particularly distinguished by the total absence of the remains of the 
Echinodermata. It contains many species peculiar to it, but is not 
characterized by the appearance of any new genera. 
The Lower Helderberg Group.—This group was named from the — 
Helderberg mountains, in New York, and was defined by Hall, in 
1859. It is composed of a series of limestone strata, and has an ex- 
tensive distribution in the eastern part of the continent. It is 500 feet 
thick in New York, and extending eastwardly it reaches its greatest 
dévelopment at Gaspe, Canada, where it is 2,000 feet inthickness, It 
occurs in Maine and in Pennsylvania, where it is 1,700 feet in thick- 
ness, and in Tennessee where it is only 100 feet thick. 
The distribution of the genera, which are supposed to have com- 
menced an existence in this group, is as follows: 
Among the plants, Annularia extends to the Coal Measures. 
Among the Polypi, Spherolites is peculiar to it. 
Among the Echinodermata, #driocrinus extends to the Upper 
Helderberg; Aspidocrinus, Brachiocrinus, Coronocrinus, Dictyo- 
crinus, Mariacrinus, and Spherocystites are peculiar to it. 
Among the Bryozoa, Ichthyorachis is peculiar to it. 
Among the Brachiopoda, Katonia, Merista and Rensseleria extend 
up into the Devonian, and Camarium is peculiar to it. 
Among the Lamellibranchiata, Mytilarca extends to the Subcar- 
boniferous; Megambonia and Ilionia to the Upper Helderberg, and 
Peteronitella is peculiar to it. 
