262 MOLLUSCA. CEPHALOPODA 



prolongation of the dorsal end of the body, containing 

 arteries, and known as the siphuncle (d). The position 

 of the siphuncle varies in different genera; in Nautilus 

 it pierces the septa at or near their centres ; in others 

 it may be near either the external or the internal 

 margin of the whorl. In the modern Nautilus the 

 siphuncle has only a thin calcareous covering; but in 

 many fossil Nautiloids it is completely invested by a 

 calcareous tube. In Palaeozoic genera the interior of the 

 calcareous siphuncle is frequently partly filled up with 

 calcareous deposits. The septa are often prolonged in the 

 form of funnels around the siphuncle, so as to more or 

 less completely insheath it ; they may be short or may 

 reach from one septum to the next or even further ; these 

 funnels are termed septal necks (c) ; in nearly all the 

 Nautiloidea they are directed backwards. 



The aperture of the shell has, in some cases, a simple 

 margin, being either straight or slightly curved ; in others, 

 processes are given off from the external margin or from 

 the sides; in Nautilus there is a sinus at the external 

 (ventral) margin where the funnel occurs, and the lines 

 of growth on the shell are correspondingly curved. In 

 some fossil Nautiloids (Phragmoceras) the sinus is at 

 the inner margin of the aperture, which was therefore 

 presumably ventral. In a few forms (e.g. Gomphoceras, 

 fig. 106) the aperture, owing to the inward growth of the 

 margin of the body-chamber, is constricted. 



The line wdiere the edge of the septum unites with the 

 outer or tubular part of the shell is known as the suture', 

 obviously this will only be seen when the shell is removed ; 

 but fossil forms frequently occur as casts and in these the 

 sutures are clearly shown. One of the chief characters 

 of the shell in the Nautiloidea is the simple form of the 



