Suze Adams

Gryphaea: related to the oyster, Gryphaea is characterized by its distinctively convoluted shape; fine markings extend across the shell at right angles to the direction of growth. In some mature specimens, the coiling of the shell became so pronounced that it is unlikely that the shell could be opened at all, at which point the animal must have died.
Edited extract from: http://www.britannica.com/EBchecked/topic/247503/Gryphaea