Fig. 10.—Paddle-fish. Polyodon Spathula (Walbaum). Ohio River.
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.
Order Pycnodonti.—In the extinct order Pycnodonti, as recognized by Dr. O. P. Hay, the notochord is persistent and without ossification, the body is very deep, the teeth are always blunt, the opercular apparatus is reduced, the dorsal fin many-rayed, and the fins without fulcra. The scales are rhombic, but are sometimes wanting, at least on the tail. Many genera and species of Pycnodontidæ are described, mostly from Triassic and Jurassic rocks of Europe. Leading European genera are Pycnodus, Typodus (Mesodon), Gyrodus, and Palæobalistum. The numerous American species belong to Typodus, Cœlodus, Pycnodus, Hadrodus, and Uranoplosus. These forms have no affinity with Balistes, although there is some resemblance in appearance, which has suggested the name of Palæobalistum.
Fig. 12.—Gyrodus hexagonus Agassiz. Family Pycnodontidæ. Lithographic Shales.
Woodward places these fishes with the Semionotidæ and Halecomorphi in his suborder of Protospondyli. It seems preferable, however, to consider them as forming a distinct order.
Fig. 13.—Mesturus verrucosus Wagner. Family Pycnodontidæ. (After Woodward.)