INTRODUCTION. 
xix 
notable number of Hamilton species recur, but in nearly all cases they appear 
in a pauperized condition, as if the sediments were unfavorable to their growth 
and development. The fauna of the group, as developed at Ithaca and vicinity, 
is comparatively limited in geographical extent, being interrupted on the east by 
the influx of coarser sediments, and, so far as observed, the strata soon thin 
out on the west, or are merged in beds above and below, losing any distinctive 
faunal character which may mark the formation in its central development. 
A considerable number of species occur in this part of the group, which do 
not pass above that horizon, and it was from this fact, and also from the recur¬ 
rence of lower forms in the same strata, that Mr. Vanuxem proposed to 
designate the formation as the Ithaca group. There are difficulties attending 
such a separation and distinction, since there is yet no well-recognized limi¬ 
tation between the fossiliferous beds of Ithaca and the Portage below, and, on 
the other hand, it has not been shown that this formation is distinctly separated 
from the typical Chemung lying above, and which has a great development in 
Chemung and adjacent counties. 
The Lamellibranchs most characteristic of the central part of the group, 
are several species of Leptodesma and Pterinea, which are abundant in the 
valley of the Chemung river and in numerous localities in the same 
horizon. 
The upper members of the Chemung series are in general well marked and 
characterized by the abundant presence of Spirifera disjunda=S. Verneuili, which 
is common to the various beds of shale, flags, slates, sandstones, conglomerates 
and calcareous bands found in the upper part of this group; but the Lamelli- 
branchiate fauna has no such general or uniform distribution. 
These fossils occur in small assemblages of species and genera, and are very 
circumscribed in extent, as if their duration or continuance was limited to each 
change in the sediments, which were apparently deposited in shallow pools or 
small areas over the sea bottom. This at least is the present aspect of the 
question, and we can only await more extended and critical examinations of 
the exposures of these strata before propounding any other solution of the 
problem. 
