520 STUART WELLER 
age, the two lower divisions of the upper Ordovician or Cincinnatian 
period, are restricted, so far as known, chiefly to the region lying east 
of the Cincinnati arch. During Richmond time, however, the inte- 
rior sea again transgressed to the west and north until it again stretched 
from Appalachia to the Rocky Mountain region. The evidence for 
this withdrawal and readvance of the interior sea in post-Trenton 
times is both paleontologic and stratigraphic in nature, and some 
portions of this evidence will be considered in the following pages. 
In the earlier work of the geologists of Illinois, Wisconsin, Iowa, 
and Missouri, no important subdivisions of the Cincinnatian, or 
‘Hudson River” as it was usually termed, were recognized, and 
consequently, in the absence of any conspicuous stratigraphic uncon- 
formity, the sequence from the Trenton limestone or its equivalent 
to the close of the Ordovician seemed to be complete. More 
recent critical investigation of the Cincinnatian faunas in the 
Ohio Valley has demonstrated that at least three distinct faunal 
divisions, the Utica, Lorraine, and Richmond, are included in the 
period, and the only one of these which has been recognized anywhere 
in Illinois and Wisconsin or in the region to the west belongs to 
the highest or Richmond division of the Cincinnatian period. 
The fauna of the Richmond in the Mississippi Valley exhibits two 
facies, or perhaps more properly two sub-stages, which are more or 
less different in character. In the Richmond fauna proper, as typi- 
fied by the fauna of the beds at Richmond Indiana, the brachiopod 
Rhynchotrema capax is by far the most characteristic species, and 
beds bearing this same fauna occur at many localities in Illinois, 
Wisconsin, and Missouri. 
In northwestern Illinois and eastern Iowa, however, a second 
faunal facies occurs in the Maquoketa shales, where an important 
element in the fauna consists of a large number of small pelecypod 
shells belonging to the genera Cleidophorus and Ctenodonta, Cleido- 
phorus neglectus being perhaps the most conspicuous species. ‘The 
fauna of the Maquoketa beds, however, is not uniform throughout, 
and faunules are frequently present in Iowa and elsewhere contain- 
ing examples of Rhynchotrema capax and other members of the 
typical Richmond fauna. At points farther south in the Mississippi 
Valley, in Jefferson County, Missouri, and Monroe County, Illinois, 
