Ichthyologists classify fish scales into a few general types:
- placoid - sharp and toothlike, found on sharks and rays
- ganoid - thick, diamond shaped, bony plates found on sturgeons and gars
- cosmoid - similar to placoid but found on primitive fishes such as the lungfish and coelocanth
- leptoid - derived from ganoid scales and consisting of a single layer of bone, occurring only in higher bony fishes. There are two forms:
- cycloid - smooth, rounded, found on most soft-fin fish (e.g., trout)
- ctenoid - similar to cycloid but with tiny, comblike projections on the exposed portion of the scale. Found mainly on fishes with spiny fins (e.g., cichlids, sunfish)