22 The Ganoids 



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 Poly- 

 odontidce. 



Order Pycnodonti. In the extinct order Pycnodonti, as rec- 

 ognized by Dr. O. P. Hay, the notochord is persistent and with- 

 out ossification, the body is very deep, the teeth are always 



FIG. 12. Gyrodus hexagonus Agassiz. Family Pycnodontidce. 

 Lithographic Shales. 



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 Pycnodontida are described, mostly from Triassic 

 and Jurassic rocks of Europe. Leading European genera are 

 Pycnodus, Typodus (Mesodon), Gyrodus, and Palceobalistum. The 

 numerous American species belong to Typodus, Ccelodus, 

 Pycnodus, Hadrodus, and Uranoplosus. These forms have no 

 affinity with Balistes, although there is some resemblance in 

 appearance, which has suggested the name of Palaobalistum. 



