Ulrich on Ccrrelalion of the Lower Silurian. loi 
the various states in which the rocks under consideration are 
exposed, a nearly conchisive correhition seems possible. Being 
especially interested in pala?ontological problems, it follows that 
stratigraphy and the lithological peculiarities of the various 
beds to be discussed, though not by any means ignored, have, 
nevertheless, been subordinated to evidence furnished by their 
fossil contents. 
In late years a disposition to undervalue j^alaeontological evi- 
dence, as determinative of the position and inter-relation of 
strata, has been making itself felt. That stratigraphists and 
lithologists have some excuse for abandoning their confidence 
in determinations based upon fossil evidence, I am w'illing to 
admit. Still, I am confident the fault lies not so much with 
the fossils as with their identifiers. What possible value can a 
determination of any of the Lower Silurian divisions have, 
which, among a total of five or six doubtful species, mentions 
several of such long-lived forms as Strophomeita alternata^ 
Zygospira modesta^ Orthis lynx^ O. testttdinario, ]\l7irchisoiiia 
bicincta, Pletirotomaria {^Raphistotna^ lenticular is .^ and Plcu- 
rotojnaria subconica P Besides these we almost invariably find 
that the lists con\.3\n Monticulipoi-a or C/icBtetes petropolitana, 
lycoperdon und^brosa, three names that together have been ap- 
plied to no less than fifty distinct species. I ask again, what 
value can an identification resting upon such grounds have? 
Practically none. 
The determination of particular stratigrapliical horizons by 
means of their characteristic organic remains is sufficiently diffi- 
cult even under the most favorable circumstances. Still geolo- 
gists have little or no excuse for faulty determinations, since, if 
correctly identified, the fossils fuinish very reliable data. In 
my opinion, at any rate, it is by far the most reliable and ready 
evidence obtainable. Species known to have great vertical range 
are of course not available for the identification of the minor 
geological divisions, except to expert paloeontologists. The}- 
are of great use to the skilled, because he is capable of noting 
every change in the development of the species, and I believe 
I can claim without fear of contradiction, that scarcely a single 
form passes from one recognizable division of rocks to another 
without sustaininsr a more or less marked chant/c. vSonietimcs 
