J. Hall on the Fossils of New York. 247 
forms including the Bryozoa, we have a number of species equal 
to either of the orders of Mollusca, while the number of crusta- 
ceans known is not more than half as great. Farther researches, 
however, in a more extended field of observation, and where the 
r nature of the sediment has influenced the character of the fauna, 
me may produce species which will change the proportions here given. 
_ ‘hese researches had shown also that there were not only spe- 
cies peculiar to the lower stage of the system, but that generic 
forms were likewise characteristic, and that certain of these as- 
sume, as it were, a gradual development or change in ascending 
the geological scale. It was therefore not always necessary to 
that the number of species of these two genera and of Atrypa, 
present scarcely a difference in number of species, while the a 
ing one, becomes numerous in the Devonian period, and like 
the bar, ‘ 
