We present a coarse-grained model of clathrin that is simple enough to be computationally tractable yet includes key observed qualitative features: a triskelion structure with excluded volume between legs; assembly of polymorphic cages in the bulk; formation of buds on a membrane. We investigate the assembly of our model using both Monte Carlo simulations and molecular dynamics with hydrodynamic interactions, in the latter employing a new membrane boundary condition. In the bulk, a range of

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... n clathrin structures are assembled. A membrane budding pathway involving the coalescence of multiple small clusters is identified.