SHELLS OF CEPHALOPODA 



Ammonites and Nautilus (Tetrabranchiata). Septa; siphuncle. 



Belemnites (extinct Dibranchiata). Phragmocone (rudiment of original chambered shell) ; rostrum 

 or guard ; proostracum. (Notice hooks on arms.) 



Sepia. Internal ' cuttle bone.' Rudimentary rostrum ; laminated region : lateral expansion. . 

 Loligo. Internal horny 'pen.' 





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