284 HENRY SHALER WILLIAMS 
the horizon it occupies in another, it is necessary to prove the 
identity of the moving fauna in its different localities; and, sec- 
ond, to have a continuous datum-horizon, either above or below, 
with which to measure its lower or higher position. Neither of 
these conditions are present in the case of the Chouteau fauna. 
Furthermore, if the Chouteau fauna of Missouri were equiva- 
lent to the Chemung, its species must have descended from 
ancestors living before the Chemung period; but if it followed 
the Chemung, it must have been descended from a fauna living 
in Chemung time. The presence in the Chouteau of such species 
as Productus hallana (=dissimilis) and Rhynchonella (Pugnax) acu- 
minata does not signify identity of age with the base of the 
New York Chemung, for two reasons. First, they belong to a 
‘part of the fauna which was directly continued up into the Car- 
boniferous. It was this peculiarity which led to designating the 
fauna containing them as having a ‘‘Carboniferous aspect.”’ The 
Hamilton fauna of New York lacked this Carboniferous aspect, 
while the middle Devonian faunas of Iowa and ot Europe pos- 
sess it, and it was this fact which suggested the interpretation of 
the Cuboides zone as evidence of the incursion of the new fauna 
from the west into the New York area." 
The second reason is that the presence of these species in 
the Chouteau does not indicate that the fauna is Devonian in 
the face of its many species of distinctly Carboniferous age, 
any more than their presence in the High Point fauna pointed 
to its Carboniferous age because Rhynchonella acuminata was a 
typical Carboniferous form. The fact that they are both con- 
tained in the typical Devonian beds of Iowa explains their pres- 
ence both in the New York and Missouri rocks, but does not 
indicate identical age. The statement about ‘‘appearing as they 
do for the first time after the removal of the land barrier’’ fur- 
nishing good ground for this correlation, ceases to be forcible 
when we put the question, how do we know anything about the 
time the barrier was removed, except through the testimony of 
«Bull. Geol. Soc. Amer., Vol. I., pp. 481-500, 1890; Amer. Jour. Sci. (3), Vol. 
XXV., pp., 97-104, 1883. 
