From the Greek kykloeides - circular, round.

Cycloid scales are bony scales with smooth, rounded edges. They sit unevenly on the skin and fall off easily, e.g. on grouse, tugun and capelin. Cycloid scales are thought to be more primitive than ctenoid scales and are found in less organised fish (herring, carp).

Characterised by a smooth rounded posterior margin. Each scale lies in a deep connective tissue pocket that overlaps the next one in a tile-like pattern (only the posterior edge is visible from the outside) and consists of a homogeneous cap and fibrous basal layers of acellular bone tissue. There is a correlation between the size of the scales, the degree of their overlap and the peculiarities of fish ecology. Cycloid scales do not change during the life of the fish.