4 
PALAEONTOLOGY OP NEW-YO&K, 
part of the State, its place is still marked by a thin band of limestone even as far east as the 
base of the Helderberg at Schoharie, and upon the Hudson river near Rondout. 
It will be seen, therefore, that these three groups, though in some parts so intimately con¬ 
nected that no well marked line of separation is perceptible, have nevertheless an independent 
existence. The Medina sandstone proper thins out, so as not to be recognized east of the central 
part of the State, The Clinton group proper has its greatest development in the central part of 
the State, extends eastward to the Helderberg in a thin bed of shale, and westward beyond the 
Niagara river in alternating beds of shale and limestone. The Niagara group preserves its inde¬ 
pendent character, though greatly diminished, eastward to the Hudson river, and, in its western 
extension, becomes more prominent than either of the others. The Medina sandstone and 
Clinton group gradually recede in character from each other in their western extension ; and 
finally the former is altogether lost, while the latter becomes assimilated in character to the 
Niagara group, so far that they are regarded as the same. 
MEDINA SANDSTONE, 
This rock has not been recognized east of Oneida county. It is traced from thence towards 
Oswego, and extends thence along the southern shore of Lake Ontario to its western extremity, 
whence it takes a northwesterly direction. It is fossiliferous at few points only; those of 
Oswego, Rochester, Medina and Lockport being the principal ones. The fossils are almost 
entirely confined to its upper portions, rarely appearing one hundred feet below its surface. 
The occurrence of peculiar marine plants is its most interesting feature. 
FOSSIL PLANTS OF THE MEDINA SANDSTONE. 
Genus ARTHROPHYCUS. 
[Gr. apOpov, articulus, and <pu fucus.~\ 
Character. Stems simple or branching, rounded or subangular, flexuous, ascending, trans¬ 
versely marked by ridges or articulations. 
The species of this genus yet known consist either of simple elongated stems of nearly equal 
dimensions throughout, or those which divide near the root into several branches, and afterwards 
remain simple. The species of the Medina sandstone may be regarded as the typical form. 
