Most fish have a covering of scales, which can be divided into a variety of types, over the outer surface of their bodies. These types include the plate-like placoid scales of sharks; the diamond-shaped ganoid scales of the gars; the thin, smooth, disk-like cycloid scales of most freshwater fish and many marine species; and the ctenoid scales (with ctenii—small projections along the posterior margins) of perches and sunfish (Casteel, 1976). All the species presented in this atlas are referable to the cycloid and ctenoid types. Distinguishable scale characteristics include (1) overall scale shape; (2) position and shape of focus; (3) circuli appearance; (4) the appearance of the lateral, anterior, and posterior fields; and (5) to some extent, thickness/robustness of the scale.

As there is considerable variation in scale shape even between different areas of the same individual fish (Figure 2), scale outline is not always the best indicator for identification (Chikuni, 1968; Casteel, 1972). Size is also generally not a desirable characteristic, as scale size varies and overlap occurs not only between species and individuals, but also within a single specimen. Cycloid (Figure 3) and ctenoid (Figure 4) scales show considerable variation in their forms, although not always at either the genus or species level, permitting their use for identification purposes. The Salmonidae and, to a lesser extent, the Pleuronectidae, display particularly consistent morphological characters. At this time identification of preserved scales to higher taxonomic levels (e.g., Family) is straightforward providing that an adequate comparative reference collection is available. While species idenfication is possible for some genera, due to the lack of distinguishing characteristics, species-level assignments for many groups are not possible.

The terminology used to describe scales usually refers to topographical features such as surface sculpturing or internal variations (Figure 3, Figure 4). Lagler (1947) established the terminology utilized here to describe various scale features as follows:

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