the shells grew within
The question was this: Why are seashells often found far from the sea, sometimes embedded in solid rock at the tops of mountains? The ancient Greeks had known and written about these seashells. Medieval theologians had noticed them in the building stones of their cathedrals. Miners and quarrymen found them, as did farmers, shepherds, and travelers. Even the Pope in Rome must have noticed them and wondered, because they littered the slopes of Vatican Hill.
Today we think it natural to say that the seashells were left by a sea that once covered the land. This, in fact, was the explanation offered by the ancient Greeks. The very earliest of the Greek philosophers, the so-called Pre-Socratics, made it the keystone of their various theories of the world, six centuries before Christ. Aristotle continued the tradition, writing that the waxing and waning of the seas were part of the world’s “vital process.” The land naturally experienced many inundations over the course of time.
Yet most educated people of Steno’s time rejected this idea. They thought instead that the shells grew within the Earth. Despite all appearances, the seashells were not actually seashells at all. No clams had ever lived inside the fossil clam shells; no seas had ever covered the mountains.