We investigate the hypothesis that our conceptual systems provide two formally distinct ways of representing categories by investigating the manner in which lexical nominals (e.g., "tree," "picnic table") and phrasal nominals (e.g., "black bird," "birds that like rice") are interpreted. Four experiments found that lexical nominals may be mapped onto kind representations, whereas phrasal nominals map onto class representations but not kind representations. Experiment 1 found that phrasal nominals, unlike lexical nominals, are mapped onto categories whose members need not be of a single kind. Experiments 2 and 3 found that categories named by lexical nominals enter into both class inclusion and kind hierarchies and thus support both class inclusion ("is a") and kind specification ("kind of") relations, whereas phrasal nominals map onto class representations which support only class inclusion relations. Experiment 4 showed that the two types of nominals represent hierarchical relations in different ways. Phrasal nominals (e.g., "white bear") are mapped onto classes that have criteria of membership in addition to those specified by the class picked out by the head noun of the phrase (e.g., "bear"). In contrast, lexical nominals (e.g., "polar bear") specify one way to meet the criteria specified by the more general kind concept (e.g., "bear"). Implications for the language-conceptual system interface, representation of hierarchical relations, lexicalization, and theories of conceptual combination are discussed. (Contains 1 table and 4 figures.)