244 Proceedings of the American Association. 
but after working two years it became apparent, from the accu- 
mulation of materials, that a change of plan must be adopted; 
and from that time more particular attention had been devoted to 
the lower division of the system, or to that part of the series ter- 
minating with the Hudson River group of New York. 
During all this time there had been a constant accumulation of 
species apparently new, and the desire to present all these as far 
as possible in the work, had caused some delay. He remarked, 
that it was not for the sake of presenting a great number of new 
species, that he had been desirous to give all these, but that the 
lower term of the system should be as fairly and equally repre- 
sented as the middle and higher portions which had been more 
thoroughly studied, and from which a greater proportional num- 
r of species had been heretofore described. When the present 
work was commenced, less than seventy species had been descri- 
bed from that portion of the system in the United States, and it 
could scarcely have been expected that this number could have 
been more than doubled within the limited area of a single state ; 
and yet there are at this time already described, 380 species, most 
of which are from New York. Beyond those described, there 
are others, in an imperfect and unsatisfactory condition, sufficient 
indred. 
alents of the lower Silurian of Europe, a greater number of spe- 
cies than were enumerated in the “ Silurian system,” at the time 
of its publication in 1839. , 
Another, and to the geologist a more important point, after the 
determination of species, was to ascertain accurately their geolo- 
gical range. Examinations of species, supposed to be identical, 
from widely separated groups, had always proved them unlike ; 
and many forms regarded as belonging to the same species, were 
only so regarded for want of sufficient examination. Continued 
investigations, instead of showing a greater number of species 
common to two or more widely separated groups of the older 
strata, had only shown us more accurate means of diserimination, 
erminated ones have been replaced by 
