THE LOWER SILURIAN PERIOD. I13 
ceras further agrees with the Pearly Nautilus in the fact that 
the partitions or septa separating the different air-chambers are 
Fig. 55.—Fragment of Orthoceras crebri- 
septum, Cincinnati Group, North America, 
Fig. 56 —Restoration of Ovrthoceras, 
the shell being supposed to be divided ver- 
of the natural size. The lower figure isa 
section showing the air-chambers, and the 
form and position of the siphuncle. (After 
Billings.) 
tically, and only its upper part being 
shown. a, Arms; /, Muscular tube 
(‘‘funnel”) by which water is expelled 
from the mantle-chamber; c, Air-cham- 
bers ; s, Siphuncle. 
simple and smooth, concave in front and convex behind, and 
devoid of the elaborate lobation which they exhibit in the 
Ammonites ; whilst the siphuncle pierces the septa either in 
the centre or near it. In the Nautilus, however, the shell is 
coiled into a flat spiral; whereas in Orthoceras the shell is a 
straight, longer or shorter cone, tapering behind, and gradu- 
ally expanding towards its mouth in front. The chief objec- 
tions to the belief that the animal of the Or¢hoceras was essen- 
tially like that of the Pearly Nautilus are—the comparatively 
small size of the body-chamber, the often contracted aperture 
of the mouth, and the enormous size of some specimens of 
* This illustration is taken from a rough sketch made by the author 
many years ago, but he is unable to say from what original source it was 
copied. 
H 
