Mastixiopsis nyssoides was an early Eocene mastixioid plant known from a hard one-seeded fruit stone. The stone protected the seed with a dense fibrous endocarp, while a thicker outer fruit layer surrounded it. Longitudinal grooves could give well-preserved fruits a strongly channelled exterior.

The London Clay fossils show that this distinctive mastixioid fruit type was already present in the early Eocene. However, pyritisation can hide the external grooves and the available London Clay material is not equally well preserved, so the species is best recognised from a combination of exterior form and internal structure rather than a single surface character.