PLATE XII 
Charts illustrating the distribution of various species of fossils. 
The following charts indicate the localities at which various species of fossils 
actually have been found. The species are designated by the letters used. The 
number of localities indicated depends not only on the former distribution of the 
species in question, but also on the extent of the areas within which at present the 
containing strata are exposed and on the amount of field work done within these 
areas. The delimitation of various areas by means of dots and other devices is 
intended merely to assist the eye by indicating on which parts of the chart it is 
worth while to look for the letters indicating the presence of fossil localities. 
Since, evidently, the same strata must extend for many miles under cover, and 
since unquestionably their ranges frequently extended across areas from which 
the containing strata have subsequently been removed by erosion, these charts in 
no sense indicate the entire former distribution of the fossils under consideration, 
but only that part of this area at present accessible and within which the search 
for the species in question has been found successful. Moreover, since the writer 
has not had the opportunity of visiting more than a small part of the exposures 
easily accessible, sometimes only one or two localities in an entire county, it is 
evident that future search will much extend our knowledge of the geographical dis- 
tribution of these species. 
Plate Xll indicates the distribution not only of Strophomena vicina, but also of 
Dinorthis ulrichi, which at numerous localities is associated with the former species, 
especially at the Cornishville horizon, but also occasionally near the base of the 
Pai-is bed. 
162 
