A surprising way to describe an old man’s forehead, having the roundness and roughness of an oyster shell.  Or might it be alluding to the sand-polished inside smoothness of the shell?

‘He was well-preserved and vigorous for his age, and had a forehead like an oyster shell.’

For an image of less well-preserved ageing, see this.

Source: Nikos Kazantzakis, The Last Temptation, trans. P.A. Bien (London: Faber and Faber, 1983), p. 45

Photo credit: Ben Stern at unsplash.com