PYCN0D0NTID2E. 
271 
longitudinal series; splenial dentition consisting of three longitu¬ 
dinal series, the innermost the largest. Neural and haemal arches 
of axial skeleton expanding and interlocking with digitations, thus 
encircling the notochord. Fin-rays robust, closely articulated and 
divided distally. Pelvic fins present; dorsal and anal fins much 
elevated in front, becoming low and fringe-like behind, the former 
occupying at least the hinder half of the back, and the latter some¬ 
what shorter, arising more posteriorly; caudal fin fan-shaped, 
having a convex hinder margin. Scales delicate, ornamented with 
tubercles or rugae, usually covering only the anterior half of the 
trunk in advance of the median fins, though sometimes partly 
extended over the caudal region; ventral ridge-scales with long, 
diverging processes extending upon the flank. 
Palseobalistum orbiculatum, Blainville. 
1796. Diodon orbicularis, G. S. Volta, Ittiolit. Veronese, p. clxviii, 
pi. xl. 
1803. Balistes , B. Faujas St. Fond, Essai de G^ologie, vol. i. p. 132, 
pi. vi. 
1818. Palceobalistum orbiculatum, H. D. de Blainville, Nouv. Diet. 
d’Hist. Nat. vol. xxvii. p. 339. 
1833-44. Pycnodus orbicularis, L. Agassiz, Poiss. Foss. vol. ii. pt. i. 
p. 17, pt. ii. p. 190. 
1856. Palceobalistum orbiculatum, J. J. Heckel, Denkschr. k. Akad. 
Wiss., math.-naturw. Cl. vol. xi. p. 229, pi. x. 
Type. Imperfect, erroneously restored fish; Paris Museum of 
Natural History. 
The type species, attaining a length of about 0*25. Maximum 
depth of trunk equalling about four-fifths of the length of the head 
and trunk without caudal fin; head with opercular apparatus occu¬ 
pying slightly more than one-quarter of the total length of the fish. 
Vertebral axis at origin of dorsal fin nearly midway between the 
dorsal and ventral borders of the fish. Teeth of inner or principal 
series of mandible much broader than long ; those of the second 
series similar in shape, but about half as large ; those of the outer 
series relatively small and hemispherical. Dorsal and anal fins not 
acuminate in front, but gradually diminishing in height backwards ; 
the dorsal fin, with 67 rays, occupying somewhat more than one- 
half of the back, the anal fin shorter, with 56 rays. Scales con¬ 
fined to the anterior half of the trunk in advance of the median 
fins. 
Form. $ Loc. Upper Eocene : Monte Bolca, N. Italy. 
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