3(14 ORTHOCERATITE. 
than that of any other multilocular shell, viz. 
from one-tenth to one-half of the diameter of the 
shell ; and often assumes a tumid form, which 
would admit of the distension of a membranous 
siphon. The base of the shell beyond the last 
plate presents a swelling cavity, wherein the 
body of the animal seems to have been partly 
contained. 
The Orthoceratites are straight and conical, 
and bear the same relation to the Nautili which 
Baculites (see PL 44, Fig. 5) bear to Ammo- 
nites ; the Orthoceratites, with their simple 
transverse septa, resembling straight Nautili ; 
and the Baculites, with a sinuous septa, having 
the appearance of straight Ammonites. They 
vary considerably in external figure, and also in 
size ; some of the largest species exceeding a 
yard in length, and half a foot in diameter. A 
single specimen has been known to contain 
nearly seventy air chambers. The body of the 
animal which required so large a float to balance 
it, must have greatly exceeded, in all its propor- 
tions, the most gigantic of our recent Cephalo- 
pods ; and the vast number of Orthoceratites 
that are occasionally crowded together in a single 
block of stone, shows how abundantly they must 
have swarmed in the waters of the early seas. 
These shells are found in the greatest numbers 
in blocks of marble, of a dark red colour, from 
the transition Limestone of Oeland, which some 
