THE LOWER SILURIAN PERIOD. 



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- 

 septutn, Cincinnati Group, North America, 

 of the natural size. The lower figure is a 

 section showing the air-chambers, and the 

 form and position of the siphuncle. (After 

 Billings.) 



Fig. 56. Restoration of Orthoceras^ 

 the shell being supposed to be divided ver- 

 tically, and only its upper part being 

 shown, a, Arms ; f, 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 Orthoceras 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. 



