=Order Selachostomi: the Paddle-fishes.=—Another type of Ganoids, allied to the sturgeons, perhaps still further degenerate, is that of the paddle-fishes, called by Cope _Selachostomi_ (σέλαχος, shark; στόμα, mouth). This group consists of a single family, _Polyodontidæ_, having apparently little in common with the other Ganoids, and in appearance still more suggestive of the sharks. The common name of paddle-fishes is derived from the long flat blade in which the snout terminates. This extends far beyond the mouth, is more or less sensitive, and is used to stir up the mud in which are found the minute organisms on which the fish feeds. Under the paddle are four very minute barbels corresponding to those of the sturgeons. The vernacular names of spoonbill, duckbill cat, and shovel-fish are also derived from the form of the snout. The skin is nearly smooth, the tail is heterocercal, the teeth are very small, and a long fleshy flap covers the gill-opening. The very long and slender gill-rakers serve to strain the food (worms, leeches, water-beetles, crustaceans, and algæ) from the muddy waters from which they are taken. The most important part of this diet consists of Entomostracans. The single American species, _Polyodon spathula_, abounds through the Mississippi Valley in all the larger streams. It reaches a length of three or four feet. It is often taken in the nets, but the coarse tough flesh, like that of our inferior catfish, is not much esteemed. In the great rivers of China, the Yangtse and the Hoang Ho, is a second species, _Psephurus gladius_, with narrower snout, fewer gill-rakers, and much coarser fulcra on the tail. The habits, so far as known, are much the same. [Illustration: FIG. 9.—Paddle-fish, _Polyodon spathula_ (Walbaum). Ohio River. ] [Illustration: FIG. 10.—Paddle-fish. _Polyodon Spathula_ (Walbaum). Ohio River. ] [Illustration: FIG. 11.—_Psephurus gladius_ Günther. Yangtse River. (After Günther.) ] _Crossopholis magnicaudatus_ of the Green River Eocene shales is a primitive member of the _Polyodontidæ_. Its rostral blade is shorter than that of _Polyodon_, and the body is covered with small thin scales, each in the form of a small grooved disk with several posterior denticulations, arranged in oblique series but not in contact. The scales are quadrate in form, and more widely separated anteriorly than posteriorly. As in _Polyodon_, the teeth are minute and there are no branchiostegals. The squamation of this fish shows that _Polyodon_ as well as _Acipenser_ may have sprung from a type having rhombic scales. The tail of a Cretaceous fish, _Pholidurus disjectus_ from the Cretaceous of Europe, has been referred with doubt to this family of _Polyodontidæ_.