Mastixia parva was an early Eocene member of Mastixiaceae known from a small, hard, one-seeded fruit stone. The endocarp protected the developing seed and included a long germination valve through which the seedling could emerge. No attached leaves or stems are known, so the size and growth form of the parent plant cannot be reconstructed from the fruit alone.

The species is important because it records Mastixia in the London Clay flora at an early stage in the history of the lineage. Its internal structure is especially informative: the seed cavity curves around a deep dorsal infold, giving it a distinctive C-shaped cross-section. These features are better preserved internally than the often weathered outer surface.