390 The American Geologist. December, i9ik> 
zoarium. And moreover the important fact remains that they 
twist from left to right. 
Rhinidictya species have very strongly acrogene narrow 
flattened branches as compared to other rnonticuliporoids, 
and they are thus the ones most suited to show the twisting 
as well as to have become heliotropic. A similar species. 
Pachydictya acuta Hall, associated with it, is somewhat less 
acrogene and is less twisted. Other related forms in which 
this phenomenon does not appear, evince some reasons there- 
for, but which need not be enumerated. Pachydictya firma 
Ulr., occurring in a higher stratum of the Ordovician has a 
twisted broad stalk, which is, however, the narrowest part of 
the zoarium. In this species the turning is from right to left, 
the reverse of P. acuta Ulr., and Rhinidictya. 
No mention of heliotropism or regular turning of any kind 
is made in the original descriptions of the species, except in 
P. firma which is said to be "usually twisted compressed 
branches," but original figures represent it clearly in Rhini- 
dictya mutabilis.'-' The species may already be generally 
known by name, or even by their characters other than those 
here specially mentioned. 
The geologic conditions are such as to indicate that the 
species lived in shallow water not far from shore. Rhinidictya 
mutabilis Ulr. and Pachydictya acuta Ulr. occur most abund- 
antly at Saint Paul, Minnesota, but also in neighboring coun- 
ties and ^n Wisconsin and Iowa in part. They are from the 
so-called Trenton here which is equivalent in time to the lower 
part of the Trenton stage in New York. At that time there 
is supposed to have been a shallow sea or bay between Min- 
nesota peninsula f and Isle Wisconsin. Saint Paul is evidently 
near the site of the old shore. This locality is now at 45 de- 
grees north latitude. The evidence afiforded by the fossils sug- 
gests that then the earth rotated and the sun shone as now. 
The Pachydictya firma Ulr., occurs at Wilmington, 
Illinois, near 41 degrees north, in the uppermost of the Cin- 
cinnati shales, and its exceptional turning from right to left. 
*Geol. Nat. Hist. Survey Minn., Final Rep'., v. 3, p. 125, pi. 7. 
tSee Geol. Wis. v. I, p. 15, and map. 
