Th ear lex of the Oi'igut of Iron Ores. — Wmelull . i:!7T 
with some of the causes tending to produce these rapid changes, 
has been full}- discussed in another place, making it unnecessary 
to take up again the habits of these organisms, detected even in 
the fossils by the intimate association of the gasteropods with 
crinoids. Concerning this phase of the question much might be 
said. The conclusions reached may Ije briefly summed up as follows : 
(1.) The gasteropod shell invariably lies over the anal opening 
of the crinoid. 
(2.) The moUusk remained in this position for a considerable 
period, probably for the greater part of life, as is shown by the 
shells on highly ornamented calyces and after the removal of them 
by the concentric grooves made on the ventral plates. 
(3.) The growing shell followed closel}- the inequalities of the 
surface upon which it rested — depressions giving rise to furrows 
and protuberances to folds or nodes. 
(4.) Shells lying simply on flat surfaces are much more de- 
pressed and proportionally Ijroader than those clinging to the 
vertical or inclined portions of calyces in which the anal opening 
is situated laterally. 
The third of these statements is perhaps best illustrated b}- 
crinoids having low interradial areas and elevated radial regions, 
and is the probable explanation of the frequent occurrence of the 
more or less distinctlj- flve-lobed calyptraean shells. Until ver}' lately 
this phenomenon has admitted of no direct causal interpreta- 
tion. 
Since the appearance of the recent papers on the lial)its of the 
ancient calyptrteans it has been inferred that these gasteropods 
were parasitic in their manner of living. This was not the case. 
For in no instance did the mollusks probably interfere at all with 
the nourishment of the crinoids, but merely fed, in part at least, 
upon refuse matter. 
CLASSIFICATION OF THE THEORIES OF THE 
ORIGIN OF IRON ORES. 
By U. \' . WiNciiEi.i,, Miiini'ai)'>lis. 
A. Mechanical. 
a. Extra-terrestrial or cosmical. 
1. Meteoric fall. [1.]* 
b. Terrestrial. 
♦Figures in brackets refer to the theories as numbered and discussed 
in "The Iron Ores of Minnesota,'" Bulletin No. 6, 3Iinn. (ieol. Sur. 
