CLASSIFICATION 39 
through all the preceding ones to the apex of the 
shell. This tube, or “shell siphuncle,” covers a 
prolongation of the mantle, the function of which 
is not quite clear. The margins of the perforation 
in each septum are produced on one side into a short 
neck: these necks in the Nautiloidea and the most 
primitive of the Ammonoidea all point backwards; 
in the other Ammonoidea they point forwards. 
Further, the margins of the septa, where they join 
the outer shell, form a simple curve in the Nauti- 
loidea, whereas the “‘suture-line”’ in the Ammonoidea 
becomes highly folded (Plate XXI., Fig. 5). Another 
feature of interest distinguishes the shells of these 
two sub-orders ; in the Nautiloidea the protoconch is 
not calcareous, and the only trace left of its existence 
is a scar on the exterior termination of the first 
chamber; in the Ammonoidea the protoconch is 
calcareous and preserved. 
The chambers of the shell of Nautilus (and pre- 
sumably the samé was true of the Ammonites) during 
the life of the animal are filled with air containing 
more nitrogen than is found in atmospheric air. 
This gives great buoyancy to the shell, and so 
permits of the animal swimming rapidly. 
In Spirula alone of living Dibranchia the shell is 
partly internal (Plate XXV., Fig. 1), and the same 
was probably the case with some fossil forms; but in 
all other members of the order the shell is completely 
internal, and frequently rudimentary, while in the 
