38© 
PALAEONTOLOGY OF NEW-YORK. 
with other Silurian strata, and containing a fauna eminently silurian in 
character. 
The Waterlime group is in many places not clearly separable from the 
upper part of the Onondaga-salt group, but is entirely above the Gypsum 
beds. The latter group, in its lower portions, is almost entirely argillaceous 
or calcareo-argillaceous in composition, forming a shale or marl; but in 
its central and upper portions it contains alternating bands of magnesian 
limestone, which is variably argillaceous and siliceous; and finally the 
magnesian character becomes developed, and forms a group of strata from 
which the hydraulic cement is everywhere obtained. Hence the name 
“Waterlime group” has been given, from the common term applied to 
distinguish hydraulic cement from ordinary lime*. 
The Tentaculite limestone was originally united with the rock below, 
under the term “Waterlime group” ; but since the former rock is a thinly 
bedded blue or black limestone, abounding in certain organic remains, 
and reaching only from the Hudson river valley to the central part of the 
State; while the other rock, characterized by its gray or drab-colored 
surface and darker interior color, and almost destitute of fossils, extends 
throughout the State and lies below the first, it was thought proper to 
separate the two formations. The Eurypterus, though known at present 
but at a few points, has thus far, with a single exception, been found 
only in the lower strata. Several years since, I received from my friend 
Ledyard Lincklaen, esquire, of Cazenovia, a specimen of grayish blue 
limestone, containing the carapace of a small Eurypterus , together with 
the little Spirifer plicutus so common in the Tentaculite limestone. The 
species is quite distinct, in the form of the part preserved, from any other 
known to me. The specimen is from the central part of the State, and 
about as far west as the known extension of the Tentaculite limestone. 
* At the time Dr. Dekay described the Eurypterus, very little was known of the sequcuce of our for¬ 
mations, and even the lithological characters were not always clearly defined, and of this rock he wrote as 
follows : “It is said to be clay slate by Dr. Mitchell; graywacke slate, calciferous sandrock. transition 
sandrock, etc. by others.” Subsequent authors have several times cited this fossil as from the Slate or 
Graywacke formation, while in truth it is in the midst of the most extensive Calcareous formation of the 
Palaeozoic strata in the Eastern United States. 
