A. Winchell on supposed New Cephalopods . [354] 5 
Huron Group, . .*... 210 feet. * 
(c) Shales, limestones and flagstones,. 180 “ 
(6) Green shale,. 10 
(a) Black bituminous shales,. 20 
Hamilton Group, .. . 55 “ 
The reexamination of the ferruginous sandstones and under¬ 
lying shales, which have been made within the last two or three 
years, have tended to confirm the opinion long entertained that 
they are the equivalents of certain formations in other north¬ 
western states whose true geological horizon has not yet been 
settled by the general consent of American geologists. The in¬ 
terest which attaches to any definite information bearing upon 
the controverted question of the exact age and equivalents of 
these rocks, would seem to justify the attempt to make known 
the facts in my possession. I accordingly submit the following 
notice of some characteristic fossils occurring in the rocks under 
consideration: 
DESCRIPTION OF CEPHALOPODS FROM THE MARSHALL AND 
HURON GROUPS OF MICHIGAN. 
Orthoceras Indianense. 
0. Indianensis Hall, 13th Rep. 1ST. Y, Regents, p. 107. 
Septate portion of shell more than 3*75 inches* in length; 
inclination of sides forming an apical angle which varies in dif¬ 
ferent specimens from 6° toll 0 ; transverse section circular; 
septa at right angles with the central siphon; ratio of depth of 
chamber to its diameter, in different specimens 2*0, 2*67, 2*71 
and 3*4:6; ratio of concavity of septum to its diameter 2*81; 
ratio of diameter of siphon to diameter of shell 5*67. Ho sur¬ 
face markings discernable on the cast. 
Localities. The most abundant Orthoceras in the Marshall 
sandstone. I have specimens from Marshall, Calhoun county, 
Holland, Ottawa county, Moscow and various other points in 
Hillsdade county, and from Hard Wood Pt. (one mile S.W. of 
Pt. au Pain Sucre) on the shore of Saginaw Bay in Huron county. 
The specimens from Michigan exhibit all the characters pub¬ 
lished, of O. Indianensis Hall, except the apertural constriction, 
with reference to which I have not been enabled to make any 
observations. The casts of this species cannot be distinguished 
from those of O. cinctum Sowerby, as defined by de Koninck 
(“Animaux Fossiles,” p. 513, pi. xliii, fig. 6; xliv, 5; xlviii, 3). 
It is equally undistinguishable from O. Goldfussanum de Kon. 
(op. cit., 510; pi. xliii, 3, 4), except that the septa of this spe¬ 
cies are separated by only about one-eighth of their diameter. 
* The measurements in the following descriptions are all given in inches. Where 
a measurement is followed by a number in parentheses, the latter is the relative di¬ 
mension, the greatest measure given in the description being assumed as 100. 
