326 University Geological Survey of Kansas. 
the most valuable for this purpose occurring in the Kansas 
‘rocks. Even a glance at the chart will also show that there 
are certain species which are of no practical value for such 
purposes, as Seminula argenteia (Shep.), Productus semi- 
reticulatus Mort., Derbya crassa Meek, Meekella striaticostata 
(Cox), and others. The unfortunate thing about it is that 
only these vigorous species are the first to invade a new terri- 
tory and sometimes occupy it practically to the exclusion of 
‘others. It may ‘also happen that the time during which favor- 
able conditions exist in a region is so short that only the more 
vigorous species succeed in entering it. 
An interesting example of this is afforded in the Ames 
limestone of West Virginia. Many of these hardy species — 
broke into this region at that time, while but few of the weaker 
ones succeeded in reaching it. For this reason it is very diffi- 
cult to correlate this limestone accurately with the Kansas 
rocks.271 | 
Aside from the two classes of fossils just mentioned there 
is a third class which is of considerable value for purposes of 
correlation in a broad way. Such species as Chonetes gran- 
ulifer Owen, Aviculopecten interlineatus M. and W., Cleiothy- 
ris orbicularis (McChes.), and others, do not characterize any 
narrow horizon or even stage, but do characterize the larger 
groups of a period in such a way as to be of considerable value 
in making broad correlations, especially intercontinental ones. 
It should be remarked in this connection that there are 
cases where the chart is known to be incomplete. For instance, 
Meek describes several species from the strata at Nebraska 
City, Neb., which are not recorded from the rocks supposed to 
be their equivalent because of the lack of positive correlation, 
the idea being to avoid possibilities of error, in so far as may 
be possible, in assigning species to horizons. Again, a species 
may seem to have a very wide range in other regions and a 
very restricted one in the Kansas rocks. This is capable of 
at least two interpretations—first, that it only had access to 
the Kansas sea for a very short time, and after retreating 
during some time of unfavorable conditions was unable to re- 
furn; and second, that it is present and has not been found. 
Then, there are species which are known to have a wider geo- 
logic range than is shown on the chart but are not known from 
exact horizons. It is deemed better not to attempt to show 
* $71. Geol. Surv. W. Va., Rep., 1906, pp. 127-130, 1907. 
