The order of Nematognathi or catfishes is characterized among the Ostariophyse by the fact that the maxillary bone is rudimental, form ing the base of a long barbel. There are no true scales, the body being naked or mailed, and about the mouth are always fleshy feelers. The multitude of species inhabit chiefly the rivers of the tropics, only the typical catfishes of the principal family of Silurida occurring in waters ventral fins. These characters are lost in all or most of the living forms.
The eels may be distributed among different orders. The Symbranchia ((iiiv together; flpirxoc, gill)— Ichthyocephali, and Holostomi — have normal fish-like jaws, and the shoulder-girdle is sometimes joined to the skull. The families are Monopteridce or rice-field eels, Symbran chida, Amphipnoide and Chilobranchicice.
The true eels or apodes have the shoulder girdle free from the skull, the premaxillaries more or less coalesced with the vomer, and the body elongate and of many vertebra. The of the United States and Europe. The families are Diplomystida., Bunocephalide, Plotosida, Chacide, Chlariide, Hypophthalmide, Argidcr, Sisorida, Pygidiide, Loricariide and Calliththyider, the members of the last three groups armed with a bony coat of mail.
The order Gymnonots contains elongate eel like fishes without dorsal fin and with the tail excessively long. The vertebra are modified, as in the preceding orders, but there is no meso coracoid arch. families are Electrophoride and Gymnotide, all river-fish of South America.
The order Scyphophori contains river-fishes of Africa in which the small mouth is at the end of a long snout. There are no pharyngeal teeth, families are Anguillavidce (extinct) with dis tinct caudal and sometimes with ventral fins, Anguillide or true eels, Congride or conger-eels, Simencheylide, Murenesocide Nettasomide, Nemichthyidce, 0 phichthyidcr, lirphidcr, Heter ocongrichr, Dysommide Enche iida (extinct), Murenicke or morays, Myrocongridor and Moringuidce.
The small order of Carencheli contains one family, Derichthyide, characterized by the snake-like neck and the structure of the jaws.
The order Lyomeri includes deep-sea eels of enormous gape, with the parts of the head very loosely joined, and with the fifth gill-arch not modified to form a pharyngeal. There are and the opercular bones are considerably modi fied. In all there is a deep cavity on each side
of the cranium, covered by a thin bony plate, the supertemporal hone. The families are Mormy ride and Gymnarchicicr.
Next we may place a long series of more or less related families, known collectively as eels. In all these the upper jaw is more or less degenerate, the ventral fins are wanting, and the shoulder-girdle has typically lost its connection with the skull. The earliest fossil eels have traces of scales, the caudal fin separ ate, and, according to Dr. 0. P. Hay, abdominal two families, Saccopharyngida. and Eurypharyn gida.
Still more aberrant is the small order of Heteromi (trepoc differing; Zipoc shoulder), the spiny eels, elongate fishes, having the shouldersgirdle detached from the head and the coracoids united in an imperforate plate. This group includes eel-like fishes of the deep sea, with spines in the dorsal fin — the families, Protonotocanthide (extinct), Notacanthide and Lipogenyide.
Another order of uncertain relationship is that of Opisthomi, with normal coracoids, spines in the dorsal fin, and the shoulder-girdle remote from the skull. It contains one family, the Mastacembelidce, small fishes from the streams of tropical Asia and Africa.
Another small order, the Xenomi strange; &lux shoulder), has the coracoids rudimentary and cartilaginous, with no basal bones or actinosts to the pectoral fin. One family, Dalliicke, containing the black-fish of the marshes of Alaska and Siberia.
In the order Honlomi the mesocoracoid arch girdle, the scales are often ctenoid, and the edge of the upper jaw is formed by the pre maxillary alone, the maxillary being always toothless.
But it is impossible to define or limit the group by any single character or group of characters. It is connected with the Isospondyli through the Haplomi, on the one hand, by transitional groups of genera which may lack any one of their characters. On the other hand, in the extreme forms. earh of these distinctive is wholly undeveloped, as is the case in all of the other groups remaining to be enumerated.
In common with the soft-rayed fishes in gen eral, the air-bladder has a persistent air-duct, the fins are without spines, the ventral fins are abdominal and the scales are cycloid.