Q16 THE JOURNAL OF GEOLOGY. 
in the dase of the Chouteau group. Both of them, or nearly 
related species, are abundant in the Devonian beds of the 
western province. In the eastern province they are unknown, 
and nothing like them occurs before the immigration of the 
western fauna. Appearing as they do for the first time after the 
removal of the land barrier between the east and west, in these 
two localities, southern Missouri and New York, they furnish 
good ground upon which to correlate the formations in which 
they occur in the two regions, making the base of the Chouteau 
group contemporaneous with the base of the Chemung group of 
New York. 
In the early reports, the Chouteau beds were referred to the 
Chemung group, but at the present time they are generally con- 
sidered as younger than the Chemung because the fauna as a 
whole is more nearly related to the succeeding Carboniferous 
than to any Devonian fauna. Continued investigation seems to 
substantiate the old view of the Devonian age of the Chouteau beds. 
While the Chouteau fauna is not the Chemung fauna, the two lived 
contemporaneously. The differences between them are geo- 
graphic in their nature and origin, rather than chronologic. 
The dividing lines between time periods such as are understood 
by the terms Devonian and Carboniferous must be taken arbi- 
trarily in their closer application, whatever may be the distinctness 
of the periods from a general point of view. It is incompatible 
with modern doctrines to conceive of a sudden change through 
which the Devonian faunas of the world became extinct and 
the Carboniferous faunas simultaneously initiated. The Carbon- 
iferous fauna must have begun to take its origin in Devonian time, 
in the midst of Devonian faunas. A plane extended around the 
world between the formations having faunas most closely allied 
to those below, and the formations whose faunal alliance is 
upward, would not be a contemporaneous division. If time 
periods are arbitrarily separated by horizontal time lines, or time 
planes, of necessity it follows that cases will be observed where 
faunas with Carboniferous alliance will have lived before the 
arbitrary Carboniferous-Devonian time line, and faunas with 
