132 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
lias been made and the character of the faunule has been fully estab- 
lished. 
In so designating the faunule the distinction between fauna and 
faunule is exhibited. We may speak of a Tropidoleptus faunule in 
the Chemung formation; this will indicate only a temporary recur- 
rence of the species and its associates in the midst of the Spirifer 
disjunctus fauna. In this case the species are not supposed to have 
stopped their existence when we pass above or below the particular 
zone in which they occur. On the other hand, when the term Tropi- 
doleptus carinatus fauna is used the term includes not only all the 
species normally associated with Tropidoleptus carinatus in its dis- 
tributional metropolis, but all the adjustments and modifications 
through which the fauna passes in the course of both its migrations 
and its geological succession, so long as the dominant species, includ- 
ing Tropidoleptus carinatus, live. 
A fauna, therefore, may be modified and have a history, and its 
integrity may be discriminated by a set of dominant species, the 
fauna preserving its integrity and identity so long (in succession) 
and so far (in distribution) as the dominant species retain their 
ascendency among their associates. On the other hand, a faunule is 
limited to a single set of conditions and to a locality of limited extent, 
and maj^ not be modified in composition without losing its identity. 
THE BIONIC TIME-SCALE. 
At the close of the paper a in which this subject of the bionic means 
of measuring geological time was first announced I gave a sample 
table of classification and nomenclature constructed on this basis and 
stated the general terms to be used in constructing such a time-scale. 
They were as follows : b 
Terms of the bionic time-scale. 
Chron. — An indefinite division of geological time. 
Geochron. — The time equivalent of a formation. 
Biochron. — The time equivalent of a fauna or flora. 
Hemera. — The technical name for a monobiochron , indicated by the preserva- 
tion of the individual characteristics of all the species of a local faunule, as shown 
by the association in the rocks of the same species in the same relative abundance, 
size, and vigor. An example is the hemera of Rhynchonella {Hypothyris) 
cuboicles. 
Epoch. — The name of a dibiochron, indicating the time equivalent of the endur- 
ance of a particular species and of the integrity of the fauna of which it is the 
dominant characteristic. An example is the Tropidoleptus carinatus epoch, which 
corresponds closely to the limits of the Hamilton formation of eastern New York. 
Period. — May be defined as a tribiochron. This is the time equivalent of the 
continuance of a genus. An example is the Paradoxides period, which corre- 
sponds to the Acadian formation of the Cambrian system. 
aThe discrimination of time values in geology: Jour. Geol., Vol. IX, 1901, pp. 570-585. 
&Loc. cit., pp 583-584. 
