Acestra differs from Loricaria in having the snout much prolonged.
Sisor.—Head depressed, spatulate; trunk depressed; tail long and thin. One short dorsal fin; anal short; ventrals seven-rayed. Head partially osseous, rough; a series of bony plates along the median line of the back; lateral line rough. Eyes very small. Mouth inferior, small, transverse, with barbels; teeth none.
A single species, S. rhabdophorus, from rivers of northern Bengal. Allied to this genus is Erethistes from Assam.
Pseudecheneis.—Adipose fin of moderate length; a short dorsal with one spine and six rays; anal fin rather short. Barbels eight. Mouth small, inferior. Head depressed, covered with soft skin above; eyes small, superior. Caudal fin forked; pectorals horizontal, with a thoracic adhesive apparatus between, formed by transverse plaits of the skin. Ventrals six-rayed.
A very small species, inhabiting the mountain-streams of Khassya; by means of the adhesive apparatus it is enabled to hold on to stones, thus preventing the current from sweeping it away. Exostoma is a similar small Siluroid from Indian mountain-streams, but without the thoracic apparatus; probably its mouth performs the same function.
b. Aspredinina.
Aspredo.—Adipose fin none; dorsal short, without pungent spine; anal very long, but not united with the caudal. Head broad, much depressed; tail very long and slender. Barbels not less than six, one of which is attached to each intermaxillary; none at the nostrils. Eyes very small. Head covered with soft skin; the anterior and posterior nostrils are remote from each other. Ventrals six-rayed.
Six species are known from Guyana; the largest grows to a length of about eighteen inches. The remarkable mode of taking care of their ova has been noticed above (p. 161, Fig. [72]). Bunocephalus, Bunocephalichthys, and Harttia, from tropical America, are other genera of this sub-family which remain to be mentioned.
VII. Siluridæ Opisthopteræ.—The rayed dorsal fin is always present, short, and placed above or behind the middle of the length of the body, above or behind the ventrals which, however, are sometimes absent; anal short. Nostrils remote from each other; if a nasal barbel is present, it belongs to the anterior nostril. Lower lip not reverted. The gill-membranes are not confluent with the skin of the isthmus: Nematogenyina and Trichomycterina.
The genera Heptapterus, Nematogenys, Trichomycterus, Eremophilus, and Pariodon, belong to this sub-family. They are small South American Siluroids, the majority of which inhabit waters at high altitudes, up to 14,000 feet above the level of the sea. In the Andes they replace the Loaches of the Northern Hemisphere, which they resemble in appearance and habits, and even in coloration, offering a striking example of the fact that similar forms of animals are produced under similar external physical conditions.