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 XXL, 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 same 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 



