= 
of the Older Formations of the United States. T 
are continued, with several others. Among the new species are 
two of the genus Isotelus, which differ essentially from the 
species of that genus in the Trenton limestone. At the same 
time, however, the species of the genus occurring in the Tren- 
ton limestone in New York, and which are rarely or never 
known in the Hudson River Group, are quite abundant in this 
position, at the west. In this instance, their continuation to a 
higher position, seems due to an increased proportion of cal- 
careous matter, and perhaps also to other more favorable con- 
ditions. The dark and highly calcareous mud of the Trenton 
limestone, appears to have been the favorite resort of these 
animals in New York, while as the deposit became more purely 
argillaceous, they disappeared. At the west, on the other 
hand, this deposition of calcareous mud continued almost to 
the close of the period. 
Of the Shawangunk, or Oneida conglomerate, we have 
little or no knowledge in the west, the only representative of 
this rock and the Medina sandstone being some impure 
sandy strata at the termination of the group last described. 
These, at all the localities examined, consist of gray calcareous 
sandstone, with anadmixture of green shaly matter in spots, 
giving to the rock in some places a speckled appearance. 
Since fossils are few in these rocks, even in New York, it may 
be passed without further notice for the present object. 
à The Clinton Group in New York is one of the most varia- 
ble assemblages of materials of any known group. In some 
. . places it consists of a large development of shales and thin 
— flagstones, sandstones and conglomerates, with oólitic iron ore, 
and a small proportion of carbonate of lime. In other places 
consists of an almost equal development of shale and lime- 
_ stone with iron ore, and again the shale or iron ore have nearly 
: or entirely disappeared. — 
At the west, this group appears to be scarcely separable from 
the limestone above, and consists almost wholly of calcareous 
matter. One of the most characteristic fossils of this group 
in New York is the Pentamerus oblongus, which, however, in 
