
Whereas human and many animals use teeth to crush or grind food as an initial part of the digestive process, some species such as birds that lack teeth grind food inside the gizzard — a structure between the mouth and the stomach — with the help of stones. Another interesting adaptation of this approach to digestion has evolved in most of the Cephalaspidean gastropods, a common type of marine mollusks, who use hardened plates that line the gizzard for crushing or grinding. Recently, scientists studied the properties of gizzard plates of the cephalaspid Philine quadripartite.
http://www.sciencedaily.com/releases/2015/10/151019104147.htm