Cycloid scales are smooth, flat, round and consist of two distinct regions. The deeper fibrous layer is mainly composed of collagen and the outer bony layer is an organic framework saturated primarily with calcium-based salts. Cycloid scales grow larger with the fish they cover, which sometimes result in growth rings that are similar to those found in trees. When weather is cooler, cycloid scales grow more slowly and the rings lie closer together, leaving a dark band known as an annulus. Biologists can estimate the age of fish by counting the annuli on its scales, a feat that can be extremely useful in scientific studies.