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Siluriformes: Catfish, Symplectic, subopercular, basihyal, and intermuscular bones absent; parietals probably present but fused to supraoccipital; mesopterygoid very reduced; preopercle and interopercle relatively small; posttemporal probably fused to supracleithrum but thought by some to be present as a separate element in many families; vomer usually toothed (as is the pterygoid and palatine); dorsal- and anal fin pterygiophores lacking middle radial ossification (as is also true for gymnotiforms), distal radial also absent in silurids; adipose fin usually present; spinelike (=spinous) rays often present at the front of the dorsal and pectoral fins (referred to as spines in family descriptions) (the dorsal fin of most catfishes technically has two spines - the first being very short and forming a locking mechanism for the second spine, which is usually the only one referred to in the family descriptions); body either naked or covered with bony plates; normally up to four pairs of barbels on head, one nasal, one maxillary, and two on chin (i.e., on the lower jaw or mandible), the nasal and chin barbels may be variously absent; maxilla toothless and rudimentary (except in Diplomystidae and the extinct Hypsidoridae), supporting a barbel; principal caudal fin rays 18 or fewer (most with 17); caudal skeleton varying between having six separate hypural plates to complete fusion of caudal elements; eyes usually small (barbels are important in detecting food); air-breathing organs in Clariidae and Heteropneustidae. In contrast to other teleosts, where the urohyal forms as an unpaired ossification of the tendon of the sternohyoideus muscle, which is equivalent to "parurohyal" in siluriforms.

Families in this order: