 Bivalves appear in the fossil record first in the early Cambrian more than 500 million years ago . The total number of living species is about 9 @,@ 200 . These species are placed within 1 @,@ 260 genera and 106 families . Marine bivalves ( including brackish water and estuarine species ) represent about 8 @,@ 000 species , combined in four subclasses and 99 families with 1 @,@ 100 genera . The largest recent marine families are the Veneridae , with more than 680 species and the Tellinidae and Lucinidae , each with over 500 species . The freshwater bivalves include seven families , the largest of which are the Unionidae , with about 700 species . 
