Williams.] GEOLOGICAL FAUNAS AND THEIR NOMENCLATURE. 25 
As the order of succession of the faunules is of great importance, 
the section should be examined from bottom to top and each fossilifer- 
ons zone noted, and faunules obtained and recorded as frequently as 
may be practicable. In practice it has been found that sections in 
the Devonian of New York and Pennsylvania are sufficiently alike 
for a radius of 5 or 10 miles to make the separate fossiliferons zones 
recognizable in the separate sections examined. As an actually con- 
tinuous section vertical^ is more satisfactory in fannal studies for 
the establishment of sequence than several short sections whose zones 
at top or bottom have to be correlated across a covered interval, it is 
desirable to make a thoroughly exhaustive section, extending through 
the formations examined, for at least every 15 or 20 miles. The local 
shorter sections will then fall into their places in relation to the 
general sections and prevent confusion of geological mutation with 
geographical variation. 
In reporting the faunules the identification of species is of first 
importance, but for study of the biological relations of the faunas as 
such the relative abundance and evident dominance of the species is 
of almost as great importance. Only thus are the intimate relations 
of the faunas to be established and their time values brought to light. 
After these two sets of facts are recorded, note should also be taken 
of the variability expressed by the species, and particularly those 
which are the dominant species of the faunule. It is by catching the 
particular characters of specific form which express variability, and 
the direction of the changes taking place in the form of the fossils, 
that genetic kinship of faunules is traced. 
By taking note of these characteristics of the faunules over terri- 
tories several hundred miles in extent, and ranging through the mid- 
dle and upper formations of the Devonian system, it has been possible 
to formulate several valuable rules for the discrimination and inter- 
pretation of fossil faunas. 
Faunules of the same formation , located together in the same general 
region, are more closely alike in constitution and proportionate abun- 
dance than those of widely separate regions. Hence it follows that a 
fauna has a local expression. The details and exact description of 
this local faunal expression can be stated in terms of relative abun- 
dance of the species constituting the faunules. 
Although over wide areas some of the species of a general fauna 
are recognized, the limited area within which the dominant species hold 
the same relative dominance in numbers over the other species may be 
clearly distinguished by the statistics of the faunules. 
By comparison of the species of the faunules in their relation of 
relative abundance a standard list of dominant species is formed, and 
the region over which this standard is preserved may be called the 
metropolis of the fauna. 
By the same method the faunules express for several numbers in 
