However , in contrast to Fig 3 , the protrusion on the base of the shell in Fig 4 is not part of the shell .
Is instead a wrought iron disc attached to the shell by the screw plug .
However , in contrast to Fig 3 , the protrusion on the base of the shell in Fig 4 is not part of the shell , but is instead a wrought iron disc attached to the shell by the screw plug .