Second Family—Polyodontidæ.
Body naked, or with minute stellate ossifications. Mouth lateral, very wide, with minute teeth in both jaws. Barbels none. Caudal fin with fulcra. Dorsal and anal fins approximate to the caudal. Four gills and a half; no opercular gill or pseudobranchia.
Polyodon (Spatularia).—The snout is produced into an exceedingly long, shovel-like process, thin and flexible on the sides. Spiracles present. Gill-cover terminating in a very long tapering flap. One broad branchiostegal. Each branchial arch with a double series of very long, fine, and numerous gill-rakers, the two series being divided by a broad membrane. Air-bladder cellular. Upper caudal fulcra narrow, numerous.
The single species, P. folium, occurs in the Mississippi, and grows to a length of about six feet, of which the rostral shovel takes about one-fourth; in young examples it is comparatively still longer.
Psephurus differs from Polyodon in having the rostral process less depressed and more conical. The gill-rakers are comparatively short, in moderate number, and distant from one another. Upper caudal fulcra enormously developed, and in small number (six).
Psephurus gladius inhabits the Yan-tse-Kiang and Hoangho, the distribution of the Polyodontidæ being perfectly analogous to that of Scaphirhynchus. It grows to an immense size, specimens of 20 feet in length being mentioned by Basilewsky. The function of the rostral process in the economy of these fishes is not yet sufficiently explained. Martens believes that it serves as an organ of feeling, the water of those large Asiatic and American rivers being too turbid to admit of the Sturgeon seeing its prey, which consists of other fishes. The eyes of Psephurus, as well as Polyodon, are remarkably small. Both fishes are used as food.
Fig. 143.—Psephurus gladius.
Allied to the Polyodontidæ, and likewise provided with a paddle-shaped production of the fore part of the head, is the fossil genus Chondrosteus, remains of which occur in the Lias.