Nelson, Gareth (Department of Ichthyology, The American Museum of Natural History, New York, New York 10024) 1974. Classification as an expression of phylogenetic relationships. Syst. Zool. 22:344–359.—Concepts of phylogenetic relationships are defined, and the possibilities of their expression in a hierarchical classification are explored. Concepts of relationships—either common-ancestor relationships or ancestor-descendant relationships, are precisely and completely expressible in classification, if descendants rather than ancestors are grouped together. Common-ancestor relationships are more efficiently expressed, and are advantageous for groups of unknown or doubtful relationship.