﻿46 
  ACTINOPTERYGII. 
  

  

  Acipenser 
  sp., 
  A. 
  S. 
  Woodward, 
  loc. 
  cit. 
  p. 
  29. 
  — 
  Red 
  Crag; 
  

   Suffolk. 
  [Pectoral 
  spines 
  ; 
  York 
  Museum.] 
  

  

  The 
  dermal 
  tubercles 
  named 
  Acipenser 
  tuberculosus 
  and 
  A. 
  molas- 
  

   sicus 
  by 
  Probst 
  are 
  Selachian 
  armour 
  (see 
  Part 
  I. 
  p. 
  89) 
  ; 
  and 
  the 
  

   so-called 
  Acipenser 
  cretaceus 
  (A. 
  Daimeries, 
  Bull. 
  Soc. 
  Roy. 
  Malacol. 
  

   Belg. 
  vol. 
  xxvii. 
  1892, 
  p. 
  xvi) 
  from 
  the 
  Senonian 
  of 
  Folx-les-Caves, 
  

   Belgium, 
  seems 
  to 
  be 
  founded 
  on 
  the 
  dermal 
  plates 
  of 
  some 
  fish 
  

   related 
  to 
  Dercctis 
  or 
  Eurypholis. 
  

  

  Family 
  POLYODONTID^E. 
  

  

  Trunk 
  elongate 
  or 
  elongate-fusiform 
  ; 
  tail 
  heterocercal. 
  Bones 
  of 
  

   the 
  cranial 
  roof 
  forming 
  a 
  discontinuous 
  shield, 
  with 
  vacuities 
  and 
  

   with 
  a 
  median 
  longitudinal 
  series 
  of 
  small 
  azygous 
  elements 
  on 
  the 
  

   rostral 
  region 
  ; 
  snout 
  much 
  produced 
  and 
  eye 
  far 
  forwards 
  ; 
  mouth 
  

   large, 
  with 
  minute 
  teeth 
  in 
  both 
  jaws 
  throughout 
  life 
  ; 
  no 
  pre- 
  

   maxilla 
  ; 
  operculum 
  and 
  suboperculum 
  present, 
  but 
  no 
  branchio- 
  

   stegal 
  rays. 
  Squamation 
  of 
  trunk 
  rudimentary 
  or 
  absent; 
  the 
  

   lateral 
  rhombic 
  scales 
  and 
  large 
  fulcral 
  scales 
  on 
  the 
  upper 
  caudal 
  

   lobe 
  robust. 
  

  

  Only 
  two 
  genera 
  of 
  this 
  family 
  are 
  recognized 
  among 
  existing 
  

   fishes, 
  namely, 
  Polyodon 
  (or 
  Spatularia) 
  from 
  the 
  Mississippi, 
  and 
  

   Psephurus 
  from 
  Chinese 
  rivers. 
  Fossil 
  remains 
  are 
  very 
  rare, 
  and 
  

   only 
  one 
  definable 
  extinct 
  genus 
  has 
  hitherto 
  been 
  discovered. 
  A 
  

   fragmentary 
  specimen, 
  however, 
  from 
  the 
  English 
  Chalk 
  may 
  per- 
  

   haps 
  represent 
  another 
  member 
  of 
  the 
  family. 
  

  

  A 
  detailed 
  description 
  of 
  the 
  skeleton 
  of 
  Polyodon 
  is 
  given 
  by 
  

   T. 
  W. 
  Bridge 
  in 
  Phil. 
  Trans. 
  1878, 
  pp. 
  683-733, 
  pis. 
  lv.-lvii. 
  A 
  

   figure 
  of 
  the 
  skull 
  is 
  also 
  published 
  by 
  R. 
  H. 
  Traquair, 
  Ganoid 
  

   Fishes 
  Brit. 
  Carb. 
  Form. 
  (Palaeont. 
  Soc. 
  1877), 
  pi. 
  vii. 
  fig. 
  1. 
  

  

  Genus 
  CROSSOPHOLIS, 
  Cope. 
  

   [Amer. 
  Nat. 
  1883, 
  p. 
  1152 
  (Crassopholis).'] 
  

  

  External 
  bones 
  unornamented 
  ; 
  rostrum 
  with 
  small 
  stellate 
  bones. 
  

   Dorsal 
  and 
  anal 
  fins 
  short-based 
  and 
  remote, 
  the 
  former 
  arising 
  in 
  

   advance 
  of 
  the 
  latter 
  ; 
  caudal 
  fin 
  inequilobate, 
  the 
  upper 
  lobe 
  pre- 
  

   dominant. 
  Scales 
  of 
  trunk 
  small 
  and 
  thin, 
  each 
  in 
  the 
  form 
  of 
  a 
  

   small 
  grooved 
  disc 
  with 
  several 
  posterior 
  denticulations, 
  arranged 
  

   in 
  oblique 
  series, 
  but 
  not 
  in 
  contact 
  ; 
  caudal 
  fulcral 
  scales 
  numerous, 
  

   broad 
  at 
  the 
  base 
  of 
  the 
  lobe, 
  but 
  becoming 
  slender 
  in 
  its 
  distal 
  

   portion. 
  

  

  