All fish scales are not alike. There are four major groups, one of which is comprised of cycloid and ctenoid scales. The ctenoid variety have a comb-like, spiny posterior edge, their name developing from the Greek word cteno, meaning comb. Conversely, cycloid scales have a smooth posterior edge as their name from the Greek cyclo, meaning circle, implies. Both types of scales, however, consist of two main regions; a rigid surface layer chiefly composed of calcium-based salts, and a deeper fibrous layer consisting mainly of collagen.