Tamesichthys decipiens was a distinctive mackerel-grade fish from the London Clay. Its lower jaw projected beyond the upper jaw, producing a strongly underbitten head, and large scales covered the opercular region. These features give the fish a specialised cranial profile, but its exact relationships within early scombriforms remain less clear than in familiar living mackerels.

The protruding lower jaw suggests a particular prey-capture geometry, potentially useful for snapping at small fishes or other nekton, although that function remains inferred. Three-dimensional skull material provides direct evidence for the unusual jaw proportions and heavy scale cover around the gill region.