﻿492 
  ACT1N0PTERYG1I. 
  

  

  superior 
  border 
  exhibit 
  a 
  deep 
  narrow 
  depression, 
  overlapped 
  by 
  

   two 
  supramaxillary 
  bones, 
  which 
  are 
  arranged 
  like 
  those 
  of 
  Pholi- 
  

   dophorus 
  and 
  the 
  Clupeoids. 
  Only 
  two 
  elements 
  have 
  been 
  observed 
  

   in 
  the 
  mandible, 
  a 
  short 
  articular 
  bone 
  behind 
  and 
  a 
  long 
  dentary 
  

   forwards 
  ; 
  these 
  meeting 
  in 
  a 
  high 
  coronoid 
  elevation, 
  which 
  is 
  

   chiefly 
  formed 
  by 
  the 
  dentary. 
  The 
  teeth 
  are 
  small 
  and 
  conical, 
  

   and 
  seem 
  to 
  have 
  been 
  more 
  or 
  less 
  clustered 
  on 
  the 
  margin 
  of 
  the 
  

   jaws. 
  The 
  cheek-plates 
  are 
  very 
  delicate, 
  but 
  appear 
  to 
  have 
  

   covered 
  the 
  whole 
  of 
  the 
  circumorbital 
  space. 
  The 
  opercular 
  

   apparatus 
  is 
  complete, 
  and 
  the 
  preoperculum 
  is 
  considerably 
  exposed, 
  

   marked 
  with 
  radiating 
  ridges. 
  Branchiostegal 
  rays 
  are 
  also 
  well- 
  

   developed, 
  but 
  no 
  gular 
  plate 
  has 
  been 
  observed. 
  The 
  branchial 
  

   arches 
  are 
  remarkable 
  on 
  account 
  of 
  the 
  great 
  size 
  of 
  the 
  gill-rakers, 
  

   which 
  are 
  smooth 
  pointed 
  stylets 
  of 
  bone, 
  slightly 
  constricted 
  near 
  

   the 
  base 
  and 
  arranged 
  in 
  very 
  close 
  series. 
  

  

  The 
  vertebral 
  centra 
  are 
  completely 
  ossified, 
  amphiccelous, 
  usually 
  

   deeper 
  than 
  broad, 
  and 
  marked 
  on 
  the 
  side 
  by 
  fine 
  transverse 
  

   striations 
  extending 
  between 
  a 
  thickened 
  rim 
  anteriorly 
  and 
  pos- 
  

   teriorly 
  ; 
  a 
  pair 
  of 
  deep 
  pits 
  on 
  the 
  upper 
  aspect 
  accommodates 
  the 
  

   neural 
  arch, 
  and 
  there 
  is 
  a 
  similar 
  pair 
  of 
  pits 
  on 
  the 
  ventral 
  

   aspect 
  for 
  the 
  insertion 
  of 
  a 
  haemal 
  arch. 
  The 
  only 
  traces 
  of 
  

   attached 
  peripheral 
  elements 
  hitherto 
  observed 
  consist 
  in 
  a 
  small, 
  

   faint, 
  rounded 
  pit 
  or 
  rugosity 
  on 
  some 
  of 
  the 
  anterior 
  centra, 
  which 
  

   may 
  have 
  supported 
  an 
  intermuscular 
  bone. 
  The 
  first 
  vertebral 
  

   centrum, 
  articulating 
  with 
  the 
  basioccipital, 
  is 
  composed 
  of 
  two 
  

   thin 
  discs 
  fused 
  together, 
  the 
  anterior 
  supporting 
  the 
  neural, 
  the 
  

   posterior 
  the 
  haemal 
  arch. 
  The 
  other 
  centra 
  exhibit 
  no 
  suture. 
  

   The 
  neural 
  spines 
  are 
  free 
  from 
  their 
  supporting 
  arches 
  in 
  the 
  

   abdominal 
  region 
  and 
  comparatively 
  robust 
  ; 
  those 
  of 
  the 
  caudal 
  

   region 
  are 
  slender 
  and 
  fused 
  with 
  the 
  delicate 
  low 
  arches, 
  which 
  

   have 
  prominent 
  zygapophyses. 
  The 
  ribs 
  are 
  short 
  and 
  delicate. 
  

  

  Oligopleurus 
  esocinus, 
  Thiolliere. 
  

  

  1850. 
  Oligopleurus 
  esocinus, 
  V. 
  Thiolliere, 
  Ann. 
  Sci. 
  Phys. 
  & 
  Nat. 
  

  

  Lyon, 
  [2] 
  vol. 
  iii. 
  p. 
  154. 
  

   1854-73. 
  Oligopleurus 
  esocinus, 
  V. 
  Thiolliere, 
  Poiss. 
  Foss. 
  Bugey, 
  

  

  pt. 
  i. 
  pi. 
  ix., 
  pt. 
  ii. 
  p. 
  21. 
  

  

  Type. 
  Nearly 
  complete 
  fish 
  ; 
  Lyons 
  Museum. 
  

  

  The 
  type 
  species, 
  attaining 
  a 
  length 
  of 
  at 
  least 
  0*45. 
  Length 
  

   of 
  head 
  with 
  opercular 
  apparatus 
  nearly 
  equal 
  to 
  the 
  maximum 
  

   depth 
  of 
  the 
  trunk 
  and 
  almost 
  one-quarter 
  of 
  the 
  total 
  length 
  of 
  the 
  

   fish 
  : 
  depth 
  of 
  trunk 
  at 
  pectoral 
  arch 
  comprised 
  nearly 
  six 
  times 
  in 
  

   the 
  total 
  length. 
  Anterior 
  abdominal 
  vertebral 
  centra 
  somewhat 
  

  

  

  