Dictionary of Ichthyology  

cycloid scale = a smooth-edged round or oval scale composed of acellular dermal bone lacking small spines on the posterior exposed edge. Typical of many Teleostei. Some cycloid scales may have a serrated margin and are then termed spinoid scales.

Plaice scale showing four zones of annual growth from Lydekker et al. "Reptiles, Amphibia, Fishes and Lower Chordata", 1912. Photograph by Brian W. Coad.

Canadian Museum of Nature