403 
PALAEONTOLOGY OF NEW-YORK. 
some parts of Virginia, that in the friable sandstone the shells are en¬ 
tirely silicified and quite free from adhering stone, so that the exterior 
markings and internal structure are perfectly preserved ; the interior 
being quite hollow, or filled only with loose sand. In these localities, not 
only do we find the cavities of large gasteropods with no more adhering 
matter than those of the Tertiary sands, but not unfrequently the delicate 
internal apparatus of the Brachiopoda is almost entirely preserved ; fur¬ 
nishing means for the determination of several genera, which the collec¬ 
tions from all other parts of the country have failed to show. 
The great changes in the physical conditions supervening at the close 
of the preceding group indicate an influence which would affect in an 
equal manner the fauna of the succeeding one, and we find accordingly 
few species passing from the Lower Helderberg group to the Oriskany 
sandstone. The changes, however, are mainly of a specific character; no 
new genera being introduced, so far as we now know, though some of them 
appear under modified forms. 
Amongst the Brachiopoda, Orthis, Strophodonta, Chonetes, Strophomcna 
and Leptcma, Spirifer, Cyrtia, Rhynchonella and Merista are genera which 
occur equally in this formation and in the preceding and succeeding ones. 
At the same time the Genera Rensselaria , Eatonia and Lcptocczlia are 
more fully developed than at any former or succeeding epoch. Of the 
Genus Leptoccelia we have one species in the Niagara group and two in 
the Lower Helderberg group, two of larger size in the Oriskany sand¬ 
stone, and one or two in the succeeding rocks. Of the Genus Eatonia we 
have four species in the rocks of the Lower Helderberg group; and we 
do not at present know more than an equal number in the Oriskany 
sandstone, though they are more abundant in the latter rock, and one 
species is larger than the largest of the Lower Helderberg species. Of the 
remarkable form Rensselaeria we have four species in the rocks of the 
Lower Helderberg group, while the genus becomes excessively developed 
in the Oriskany period ; not so much, however, in the number of species, 
as in their large size and the great number of individuals. We already 
know three species of this genus in rocks above the Oriskany sandstone. 
