=THE Nematognathi.=—The _Nematognathi_ (νῆμα, thread; γνάθος, jaw), known collectively as catfishes, are recognized at once by the fact that the rudimentary and usually toothless maxillary is developed as the bony base of a long barbel or feeler. Usually other feelers are found around the head, suggesting the "smellers" of a cat. The body is never scaly, being either naked and smooth or else more or less completely mailed with bony plates which often resemble superficially those of a sturgeon. Other distinctive characters are found in the skeleton, notably the absence of the subopercle, but the peculiar development of the maxillary and its barbel with the absence of scales is always distinctive. The symplectic is usually absent, and in some the air-bladder is reduced to a rudiment inclosed in a bony capsule. In almost all cases a stout spine exists in the front of the dorsal fin and in the front of each pectoral fin. This spine, made of modified or coalescent soft rays, is often a strong weapon with serrated edges and capable of inflicting a severe wound. When the fish is alarmed, it sets this spine by a rotary motion in its socket joint. It can then be depressed only by breaking it. By a rotary motion upward and toward the body the spine is again lowered. The wounds made by this spine are often painful, but this fact is due not to a specific poison but to the irregular cut and to the slime of the spine. In two genera, _Noturus_ and _Schilbeodes_, a poison-gland exists at the base of the pectoral spine, and the wound gives a sharp pain like the sting of a hornet and almost exactly like the sting of a scorpion-fish. Most of the _Nematognathi_ possess a fleshy or adipose fin behind the dorsal, exactly as in the salmon. In a few cases the adipose fin develops an anterior spine and occasionally supporting rays. All the _Nematognathi_ are carnivorous bottom feeders, devouring any prey they can swallow. Only a few enter the sea, and they occur in the greatest abundance in the Amazon region. Upward of 1200 species, arranged in 150 genera, are recorded. They vary greatly in size, from two inches to six feet in length. All are regarded as food-fishes, but the species in the sea have very tough and flavorless flesh. Some of the others are extremely delicate, with finely flavored flesh and a grateful absence of small bones. =Families of Nematognathi.=—According to Dr. Eigenmann's scheme of classification,[11] the most primitive family of Nematognathi is that of _Diplomystidæ_, characterized by the presence of a well-developed maxillary, as in other soft-rayed fishes. The single species, _Diplomystes papillosus_, is found in the waters of Chile. Footnote 11: A Revision of the South American Nematognathi, 1890, p. 7. Similar to the _Diplomystidæ_ in all other respects is the great central family of _Siluridæ_, by far the most numerous and important of all the divisions of _Nematognathi_.