A kind of fish scale, common to most modern bony fishes (order Osteichthyes), and consisting of a single layer of bone. Leptoid scales are believed to have been derived from the ganoid scales of primitive bony fish (such as gars and sturgeons) by the loss of the ganoin layer.
Leptoid scales occur in two forms: cycloid (circular) and ctenoid (toothed), the latter bearing tiny comb-like projections. The single-layer cosmoid scale found on lungfishes may also be classified as leptoid, although of a different derivation. (This is debatable, however.)