Pearl formation is generally known to occur in the pelecypod families of the pearl oysters (Pteriidae) and the freshwater pearl mussels (Margaritanidae, Unionidae). In these families precious and valuable pearls are found, consisting of thin layers of aragonite, which show iridescent colours called the orient or lustre of the pearl. According to the situation of pearl formation within the mollusc two main types of pearls are recognized: free pearls and blister pearls. Free pearls are formed within the soft parts of the animal; blister pearls are found fixed to the inside of the shell. Except for the three families mentioned above, pearl formation is known from a number of other pelecypod families: Arcidae, Glycymeridae, Mytilidae, Pinnidae, Pectinidae, Spondylidae, Anomiidae, Ostreidae, Veneridae, Tellinidae, Tridacnidae, Mactridae, and Myidae (Korschelt, 1912: 113-117; Schmidt, 1928: 143; Bolman, 1941: 5-9). Recently Alagarswami (1965) described a pearl from the venerid Gafrarium tumidum Röding. Most of the pearls from these families consist of calcite, are without iridescence, and have a porcellaneous texture.