Williams] FA UNA L DISSECTION OF THE DEVONIAN. 61 
recurrence frequency, is to exalt the rank of the species Amboccdia 
umbonata, Athyris spiriferoides and Phacops rana, and this throws 
Nucula and Nuculites to the end of the list. This result may be 
attributed to the influence of environmental conditions upon the 
species, for the conditions are more favorable for lamellibranchs in 
the eastern region, and more favorable for trilobites in the western. 
It is, secondly, traceable to the rarity of these species in the localities 
in the counties of Otsego, Delaware, Schoharie, and Albany, which 
lowers their frequency percentage for the whole area. Their fre- 
quency in Madison and Chenango counties, and again in Greene, 
Ulster, and Orange counties, and across the State line in Pennsyl- 
vania, Avould entitle them to the prominence they hold in the list as 
furnished by the other evidence. 
I conclude from the balancing up of the various kinds of evidence 
now in hand that the last list (Table V) contains the twelve most 
characteristic species of this fauna as it appears in the New York 
province, and shows the order of approximate rank they occupy in the 
fauna as a whole. 
Examination of the faunas in the formations succeeding the Hamil- 
ton formation of the eastern division of New York reveals the fact 
that this typical Tropidoleptus fauna continued to appear above the 
strict limits of the formation, though associated with new forms dis- 
tinct from those of the Tropidoleptus fauna. 
The Hamilton formation is regarded as terminating where theTully 
limestone comes in, when it is present, and where the Genesee shale 
appears, when the former is wanting. When neither of these litho- 
logical formations is present, the position in the strata was traced 
from place to place with great care by the lithological character of 
the strata with the aid of structure and minute discrimination of the 
faunal contents. The faunas confirm the accuracy of the geological 
work of Professor Prosser, and of the dissection of the local sections 
made by him. I have examined his reports with critical scrutiny, 
and have great confidence in the interpretation of the equivalency of 
the species and faunas made by him. The evidence of change in the 
faunas is clear, and the relative order of the succession of the faunas 
is always the same, and the gradual departure of the less conspicuous 
elements of the earlier fauna is apparent as the faunas are traced 
upward in each section. 
The Ithaca formation is succeeded by the Oneonta, and above the 
Oneonta a considerable number of the typical species of the Tropido- 
leptus fauna still appear. These species continue after the introduction 
of Spirifer meslcostalis and after the Spirifer mesistrialis fauna was 
well established in the province. The Tropidoleptus fauna was not 
entirely dispersed till the characteristic Spirifer disjunctus of the 
Chemung had arrived in central New York. In the extreme eastern 
counties this species is not certainly reported, but many of its asso- 
ciates in the western part of the basin are introduced before the entire 
