Trigonioids represent the most plausible marine outgroup for freshwater palaeoheterodonts, yet details of their early ontogeny are scant. We utilized museum specimens and scanning electron microscopy to characterize the prodissoconch structure of the ‘living fossil’ Neotrigonia margaritacea. This species produces a large inflated prejuvenile shell (mean length ±SD=664±16.9 μm, N=8) that lacks both an umbone and a prodissoconch II. It is concluded that N. margaritacea does not rely on planktotrophy to fuel its prejuvenile ontogeny.