Schaeffer, Bobb American Museum of Natural History, New York, New York.
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The subclass within the Chondrichthyes (cartilaginous fishes) that includes the sharks (Selachii) and the skates and rays (Batoidea). According to traditional classifications, the Chondrichthyes is divided into two subclasses: the Elasmobranchii (sharks, skates, and rays) and the Holocephali (chimaeras or ratfishes). It is probable that both groups arose independently during the Silurian or Early Devonian from a group of extinct armored fishes, the Placodermi. Members of the Elasmobranchii (elasmobranchs) are distinguished by separate gill openings, amphistylic or hyostylic jaw suspension, and sensory organs (ampullae of Lorenzini) in the head region. Characters shared with the holocephalans include a variably calcified cartilaginous endoskeleton, placoid scales, a urea retention mechanism, clasper organs in the male for internal fertilization, and the absence of an air (swim) bladder. See also: Batoidea; Chimaeriformes; Chondrichthyes; Copulatory organ; Placodermi; Scale (zoology); Selachii; Swim bladder
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