﻿PYCNODONTID^. 
  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 
  rugaa, 
  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. 
  

  

  Palaeobalistum 
  orbiculatum, 
  Blainville. 
  

  

  1796. 
  Diodon 
  orbicularis, 
  G. 
  S. 
  Volta, 
  Ittiolit. 
  Veronese, 
  p. 
  clxviii, 
  

  

  pi. 
  xl. 
  

   1803. 
  Balistes, 
  B. 
  Faujas 
  St. 
  Fond, 
  Essai 
  de 
  G6ologie, 
  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. 
  CI. 
  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. 
  

   Yertebral 
  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. 
  

  

  Not 
  represented 
  in 
  the 
  Collection. 
  

  

  