The neural spines of the coalescent centra, which form the apparently single first vertebra, concur also in sustaining the nuchal plate-armour and the first great dorsal spine. They carry the interneurals, are joined to them by suture, and one of them is often inclined towards the occiput to assist in sustaining the head; in fact, this part of the skeleton is constructed to give firm mutual support.
The shoulder-girdle of the Siluroids is also formed to give resistance to the strong weapon with which it is frequently armed. The post-temporal, as we have said above, is often united by suture to the cranium, and it obtains support below by one or two processes that are fixed on the basioccipitals and on the diapophysis of the first vertebra.
In most osseous fishes the clavicle completes the lower key of the scapular arch in joining its fellow by suture or synchondrosis without the intervention of the coracoid; but in the Siluroids the coracoid descends to take part in this joint, and sometimes even to occupy the half of the suture, which is not unfrequently constructed of very deep interlocking serratures. The solidity of this base of the pectoral spine is further augmented by the intimate union of the coracoid and scapula, which often extends to junction by suture, or even to coalescence; and these bones, moreover, give off two bony arches—the first a slender one, arising from the salient edge of the coracoid near the pectoral fin, and going to the interior face of the scapular that is applied to the interior surface of the ascending branch of the clavicle; the second and broader supplementary arch is often perforated by a large hole; it also emanates from the same salient edge of the radius, but proceeds in opposite direction to the inferior edge of the clavicle, a little before the insertion of the pectoral spine. The two arches give attachments to the muscles that move this spine; in the Synodontes and many Bagri the upper arch remains in a cartilaginous or ligamentous condition, while in Malapterurus it is the lower arch that does not ossify, but both are fully formed in the Siluri and many other Siluroids more closely allied to that typical genus. The postclavicle is also wanting in the Siluroids. The pterygoid and entopterygoid are reduced to a single bone, the symplectic is wholly wanting, and the palatine is merely a slender cylindrical bone. The sub-operculum is likewise constantly absent in all the Siluroids.
The great number of different generic types has necessitated a further division of this family into eight subdivisions:
I. Siluridæ Homalopteræ.—The dorsal and anal fins are very long, nearly equal in extent to the corresponding parts of the vertebral column.
a. Clariina.
Clarias.—Dorsal fin extending from the neck to the caudal, without adipose division. Cleft of the mouth transverse, anterior, of moderate width; barbels eight; one pair of nasal, one of maxillary, and two pairs of mandibulary barbels. Eyes small. Head depressed; its upper and lateral parts are osseous, or covered with only a very thin skin. A dendritic accessory branchial organ is attached to the convex side of the second and fourth branchial arches, and received in a cavity behind the gill-cavity proper. Ventrals six-rayed; only the pectoral has a pungent spine. Body eel-like.
Twenty species from Africa, the East Indies, and the intermediate parts of Asia; some attain to a length of six feet. They inhabit muddy and marshy waters; the physiological function of the accessory branchial organ is not known. Its skeleton is formed by a soft cartilaginous substance covered by mucous membrane, in which the vessels are imbedded. The vessels arise from branchial arteries, and return the blood into branchial veins. The vernacular name of the Nilotic species is “Carmoot.”
Heterobranchus differs from Clarias only in the structure of the dorsal fin, the posterior portion of which is adipose.
The geographical range of this genus is not quite co-extensive with that of Clarias, inasmuch as it is limited to Africa and the East-Indian Archipelago. Six species.