Chondrichthyes (Chondrichthyans)
Chondrichthyes (play /kɒnˈdrɪkθi.iːz/; from Greek χονδρ- chondr- 'cartilage', ἰχθύς ichthys 'fish') or cartilaginous "fishes" are jawed fish-like animals with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone. The class is divided into two recent subclasses: Elasmobranchii (sharks, rays and skates) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class). Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates, the extant members of which all fall into Teleostomi. Their diversity in the geological past was, however, comparatively greater. Their first representatives appeared already in the early Silurian, being represented by evolutionary very successful group - Acanthodians which quickly swamped not only seas but (in Carboniferous) sucessfuly inhabited also fresh waters.
On-line reference: Ohio State University  Img. 110: A reconstruction of Acanthodian Machaeracanthus bohemicus
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Virtual museum of the Czech Geological Survey, www.geology.cz, (C) Czech Geological Survey, 2011, v.0.99 [13.12.2011]