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 0. 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. Rept. State Geologist. Expl. pi. (US) 2, figs. 11-15. 1SSC. 



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. 



