The verbal suffixes of Seri (a language isolate of Sonora, Mexico) divide the lexicon into classes of unparalleled complexity. The paradigm has only four forms, which mark subject number and aspect (or event number), yet there are over 250 distinct types in a corpus of just under 1,000 verbs. This relation of forms to types means that by information-theoretic measures this is among the most complex inflection class systems yet studied. In part this complexity is due to the sheer wealth of allomorphs and the freedom with which they combine within the paradigm; however, these properties can be found in all inflection class systems of any complexity. The unique property of Seri is that although the suffix morphology and the morphosyntactic paradigm have the same featural content, the two systems are not directly coordinated. Both suffix morphology and verbal morphosyntax are based on the concatenation of markers of plurality, and an increase in the morphological marking of plurality reflects a morphosyntactic accumulation of subject and predicate plurality (i.e. aspect). In this sense, morphology is a direct exponent of featural content. But there is no consistent mapping between the two systems, and the precise calibration between morphological form and morphosyntactic function must be lexically specified; it is this specification that increases dramatically the number of inflectional types. Seri therefore represents a middle ground between the conceptual extremes of morphosyntactically motivated and morphologically autonomous morphology that serve as a basis for much of our theory building.