Septum -LRB- marine biology -RRB- : walls between each chamber , or siphuncle , in shells of nautiloids , ammonites , and belemnites ; i.e. cephalopods that retain an external shell .

Cephalopod Septa : walls between each chamber , or siphuncle , in shells of nautiloids , ammonites , and belemnites ; i.e. cephalopods that retain an external shell .
