Cosmoid scales are found in the Lungfishes (family Ceratodidae) and some fossil fishes.
Cosmoid scales are similar to placoid scales and probably evolved from the fusion of placoid scales. They consist of two basal layers of bone, a layer of dentine-like cosmine, and an outer layer of vitrodentine.
As the fish grows each scale becomes larger as new bone is added to the basal layers.
Helfman, G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.