172 MOLLUSC A. CEPHALOPODA. 



a number of chambers by means of partitions termed 

 septa (fig. 49 6); the size of the chambers increases 

 towards the aperture of the shell. The body of the animal 

 was placed in the last chamber (a), all the earlier ones 

 being filled with air only. Originally the earlier chambers 

 were occupied by the animal, but as growth went on, it 

 successively withdrew to the larger parts of the shell, each 

 time depositing behind it a septum and thus forming a 

 new chamber. All the chambers are traversed by a tube 

 known as the siphunde (d), which is given off from the 

 posterior part of the animal. The position of the siphuncle 

 varies in different genera ; in some (Nautilus) it pierces 

 the septa at or near their centre, in others (Ammonites) 

 at the external margin, in others (Clymenia) at the internal 

 margin. In the modern Nautilus the siphuncle is mem- 

 branous, except that it is coated with calcareous granules ; 

 but in some fossil forms it was completely invested by a 

 calcareous tube. In addition to this the septa are fre- 

 quently produced in the form of funnels around the 

 siphuncle, so as to more or less completely eusheathe it. 

 These funnels are termed septal necks (c) ; in some genera 

 (Nautilus) they are directed backwards, in others (Ammo- 

 nites) forwards. 



In the genus Nautilus, the outer convex portion of the 

 shell is the ventral side, and the inner the dorsal ; but in 

 some fossil forms the opposite was the case, the outer 

 being dorsal, and the inner ventral. It will therefore be 

 more convenient to speak of the external and internal 

 margins, instead of the dorsal and ventral. The margin 

 of the aperture is in some cases even all round ; in others 

 a process is given off from the external margin, or the 

 lateral margins are produced into ' ears,' or the external 

 margin may possess a sinus. In a few forms the aperture 



