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a subclass of cartilaginous fishes. The skeleton is cartilaginous and often calcified. The body is covered with placoid scales or, sometimes, is naked. The gills are lamellar. There is no gill cover; five to seven pairs of gill clefts open directly to the exterior. The mouth is inferior. The intestine has a spiral fold and opens into the cloaca. In males the terminal rays of the ventral fins are converted into copulatory organs, or pterygopodia. The subclass comprises two extinct orders and one extant order, the Selachii, which includes sharks (three suborders) and rays. Elasmobranchii first appeared in the Devonian. Most of the fishes inhabit seas; a few are encountered in fresh waters.