Percostoma angustum is a taxonomically uncertain acanthomorph from the London Clay. Its skull is laterally compressed and carries a very large supraoccipital crest, producing a high, narrow head profile. Those features indicate a specialised cranial architecture, but they do not securely place the fish within a modern family or reveal a specific diet.

The species is therefore best treated as an example of early acanthomorph morphological diversity. A complete ecological reconstruction would go beyond the preserved evidence; the known material chiefly documents the neurocranium, crest and orbital region.