258 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- 
odontide. 
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 
Fie. 195.—Gyrodus hexagonus Agassiz. Family Pycnodontide. 
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 Pycnodontide are described, mostly from Triassic 
and Jurassic rocks of Europe. Leading European genera are 
Pycnodus, Typodus (Mesodon), Gyrodus, and Paleobalistum. The 
numerous American species belong to Typodus, Celodus, 
Pycnodus, Hadrodus, and Uranoplosus. These forms have no 
affinity with Balistes, although there is some resemblance in 
appearance, which has suggested the name of Paleobalistum, 
