rays. The gill-plate, being furnished with a membranous margin, can close the aperture so accurately as to exclude water, and even air. The body is long, and tapering gradually to the tail, which is furnished with a caudal fin of remarkable structure. It is unequally forked, the upper lobe being considerably the longer; but this is not the only difference, for (as in the Shark, which exhibits the same form of the caudal,) the upper lobe is penetrated by the terminal joints of the spinal column, which run through its centre to the extremity; the lower lobe is formed only of rays. The body and the head are covered with large bony plates, those on the head of various angular forms, fitting into each other, those on the body arranged in longitudinal rows, with their centres rising into spines pointing backwards. The mouth, situated beneath the head, is small and toothless; it is placed on a sort of foot of three joints, by means of which it is capable of considerable protrusion.
These are fishes of large size, some, indeed, attaining gigantic dimensions; they inhabit rivers, lakes, and inland seas, and chiefly in the northern regions of the globe. They migrate at certain seasons to the sea, but deposit their spawn in freshwater. Twenty-four species are enumerated by the Prince of Canino as belonging to the Family, including one or two singular species of North America, which, with most of the characters above-mentioned, have the snout prolonged into a broad, leaf-like, bony plate.
