CEPHALOPODA. 
31 
Septa thin, moderately concave. Sutures straight, not deeply impressed. 
Siphuncle sub-central. The elements of the siphuncle have not been 
observed, except at its passage through the septa, where it is very small, 
having a diameter of not more than 1 mm. where the tube measures 15 mm. 
in diameter. 
Test not preserved. 
Internal mould smooth. Septa marked with an areola around the 
siphuncle and a concentrically marked, ovate, organic deposit extending from 
the siphuncle to the ventral walls of the air-chambers. 
The chamber of habitation of a specimen has a length of 54 mm., and a 
diameter of 14.5 mm. at the last septum. A fragment containing nine air- 
chambers and the base of the outer chamber measures 29 mm. in the length 
of the septate portion, and is 9 mm. in diameter at the smaller extremity. 
This species may be distinguished from O. Icarus, by its sub-central siphuncle, 
the comparatively deeper air-chambers, and the more marked constriction of 
the tube near the aperture. 
Distribution. In the Goniatite limestone, at Rockford, Indiana. 
Orthoceras Icarus. 
PLATE CXVIII, FIGS. 11-15. 
Orthoceras Icarus, Hall. Fifth Ann. Kept. State Geologist. Expl. pi. (118) 2, figs. 11-15. 1886. 
The specimens of this species bear so close a resemblance to those of 0. 
Indianensis, that a statement of differences will be more satisfactory than a 
detailed description. 
In this species the air-chambers are comparatively much shallower than in 
0. Indianensis, the siphuncle is excentric, and in the specimens observed there 
is a greater amount of organic deposit surrounding it. The internal mould of 
a small fragment shows a line of strong nodes along the ventral side of the tube. 
A chamber of habitation has a length of 53 mm. with a diameter at the 
aperture of 23 mm., and of 18 mm. at the last septum. 
Distribution. In the Goniatite limestone, Rockford, Indiana. 
