Like nautiluses, to whom they were distantly related, ammonites constructed spiral shells divided into multiple chambers. The animals themselves occupied only the last and largest chamber; the rest were filled with air, an arrangement that might be compared to an apartment building in which just the penthouse is rented. The walls between the chambers, known as septa, were fantastically elaborate, folded into intricate ruffles, like the edges of a snowflake.
.c1
