Langtonia bisulcata was an early Eocene mastixioid plant known from elongate fruits with two seed chambers, although one chamber was often reduced or abortive. The woody endocarp formed a protective case around the seeds and opened along long germination valves.

Its most unusual feature is hidden inside the fruit: each seed cavity has two dorsal infolds, creating an epsilon-shaped outline in cross-section. This distinctive architecture is unlike the simpler curved locules of other London Clay mastixioids and provides a strong anatomical clue even when the outer fruit tissues have been lost.