Page:An Introduction to the Study of Fishes.djvu/71

Rh forms between the cycloid and ctenoid types. Both kiads of scales may occur not only in species of the same genus of fishes, but in the same fish.

Ganoid scales are hard and bony, covered with a layer of enamel; they are generally rhombic or quadrangular, rarely rounded and imbricate; and arranged in oblique rows, those of one row being linked together by an articulary process. This type of scales, common in fossil Ganoid fishes, occurs among recent fishes in Lepidosteus and Polypterus only.