=The Classification of the Bony Fishes.=—The classification of fishes has been greatly complicated by the variety of names applied to groups which are substantially but not quite identical one with another. The difference in these schemes of classification lies in the point of view. In all cases a single character must be brought to the front; such characters never stand quite alone, and to lay emphasis on another character is to make an alteration large or small in the name or in the boundaries of a class or order. Thus the _Ostariophysi_ with the _Isospondyli_, _Haplomi_, and a few minor groups make up the great division of the _Abdominales_. These are fishes in which the ventral fins are abdominal, that is, inserted backward, so that the pelvis is free from the clavicle, the two sets of limbs being attached to different parts of the skeleton. Most of the abdominal fishes are also soft-rayed fishes, that is, without consecutive spines in the dorsal and anal fins, and they show a number of other archaic peculiarities. The Malacopterygians (μαλακός, soft; πτερύξ, fin) of Cuvier therefore correspond very nearly to the _Abdominales_. But they are not quite the same, as the spiny-rayed barracudas and mullets have abdominal ventrals, and many unquestioned thoracic or jugular fishes, as the sea-snails and brotulids, have lost, through degeneration, all of their fin-spines. In nearly but not quite all of the Abdominal fishes the slender tube connecting the air-bladder with the œsophagus persists through life. This character defines Müller's order of _Physostomi_ (φυσός, bladder; στόμα, mouth), as opposed to his _Physoclysti_ (φυσός, bladder; κλεῖστός, closed), in which this tube is present in the embryo or larva only. Thus the _Thoracices_ and _Jugulares_, or fishes having the ventrals thoracic or jugular, together correspond almost exactly to the Acanthopterygians, (ακανθα, spine; πτερύξ, fin), or spiny-rayed fishes of Cuvier, or to the _Physoclysti_ of Müller. The Malacopterygians, the _Abdominales_, and the _Physostomi_ are in the same way practically identical groups. As the spiny-rayed fishes have mostly ctenoid scales, and the soft-rayed fishes cycloid scales, the _Physostomi_ correspond roughly to Agassiz's _Cycloidei_, and the _Physoclysti_ to his _Ctenoidei_.