﻿OLIGOrLETTRID^:. 
  491 
  

  

  The 
  undefined 
  generic 
  name 
  of 
  Holochondrv.s 
  is 
  mentioned 
  by 
  

   Thiolliere 
  immediately 
  after 
  Oligopleurus 
  (Poiss. 
  Foss. 
  Bugey, 
  pt. 
  i. 
  

   1854, 
  p. 
  4), 
  but 
  nothing 
  is 
  known 
  of 
  the 
  fish 
  to 
  which 
  it 
  refers, 
  

  

  Genus 
  OLIGOPLEURUS, 
  Thiolliere. 
  

   [Ann. 
  Sci. 
  Phys. 
  & 
  Nat. 
  Lyon, 
  [2] 
  vol. 
  iii. 
  1850, 
  p. 
  154.] 
  

  

  Head 
  large 
  and 
  snout 
  pointed 
  ; 
  maxilla 
  arched, 
  with 
  convex 
  oral 
  

   border 
  ; 
  dentary 
  truncated 
  in 
  front 
  and 
  mandible 
  prominent 
  ; 
  all 
  

   teeth 
  minute. 
  Gill-rakers 
  large 
  and 
  closely 
  arranged. 
  Vertebral 
  

   centra 
  in 
  part 
  deeper 
  than 
  long, 
  longitudinally 
  striated, 
  but 
  without 
  

   lateral 
  pits 
  ; 
  ribs 
  delicate. 
  Pin-rays 
  robust, 
  all 
  closely 
  articulated 
  

   and 
  divided 
  at 
  some 
  distance 
  from 
  the 
  base 
  ; 
  fulcra 
  conspicuous 
  on 
  

   the 
  median 
  fins. 
  Dorsal 
  and 
  anal 
  fins 
  short-based, 
  nearly 
  or 
  com- 
  

   pletely 
  opposed 
  ; 
  caudal 
  fin 
  gently 
  excavated 
  at 
  its 
  hinder 
  border. 
  

   Scales 
  large. 
  

  

  The 
  cranial 
  osteology 
  of 
  this 
  genus 
  is 
  imperfectly 
  known 
  and 
  best 
  

   illustrated 
  by 
  the 
  specimens 
  enumerated 
  below 
  from 
  the 
  Wealden 
  

   of 
  the 
  Isle 
  of 
  Wight 
  and 
  the 
  Purbeckian 
  of 
  Swanage 
  l 
  . 
  The 
  head 
  is 
  

   of 
  triangular 
  shape 
  and 
  much 
  laterally 
  compressed. 
  In 
  Oligopleurus 
  

   vectensis 
  the 
  cranial 
  roof 
  is 
  broadest 
  at 
  the 
  occipital 
  border 
  and 
  

   gently 
  arched 
  from 
  side 
  to 
  side 
  ; 
  it 
  exhibits 
  a 
  longitudinal 
  median 
  

   depression 
  attaining 
  its 
  maximum 
  depth 
  and 
  greatest 
  breadth 
  

   immediately 
  behind 
  the 
  interorbital 
  region, 
  and 
  gradually 
  becoming 
  

   shallower 
  in 
  front 
  and 
  behind. 
  There 
  is 
  no 
  ossified 
  interorbital 
  

   septum, 
  and 
  the 
  middle 
  portion 
  of 
  the 
  parasphenoid 
  is 
  narrow, 
  

   apparently 
  without 
  teeth. 
  The 
  mandibular 
  suspensorium 
  is 
  some- 
  

   what 
  inclined 
  forwards, 
  but 
  the 
  gape 
  of 
  the 
  mouth 
  is 
  wide, 
  the 
  

   mandible 
  projecting 
  in 
  front 
  of 
  the 
  premaxilla. 
  The 
  hyoman- 
  

   dibular 
  is 
  much 
  constricted 
  mesially, 
  laterally 
  compressed 
  above, 
  

   and 
  antero-posteriorly 
  compressed 
  below. 
  The 
  quadrate 
  is 
  fan- 
  

   shaped, 
  constricted 
  immediately 
  above 
  the 
  articular 
  condyle, 
  and 
  

   with 
  a 
  cleft 
  near 
  its 
  hinder 
  border 
  for 
  the 
  reception 
  of 
  the 
  sym- 
  

   plectic 
  ; 
  there 
  is 
  also 
  a 
  curious 
  inwardly-directed 
  process 
  from 
  the 
  

   base 
  of 
  the 
  condyle. 
  There 
  are 
  the 
  usual 
  three 
  pterygoid 
  elements 
  

   on 
  each 
  side, 
  but 
  rather 
  delicate. 
  The 
  premaxilla 
  is 
  very 
  small. 
  

   The 
  maxilla 
  is 
  a 
  long, 
  narrow, 
  laterally 
  compressed, 
  and 
  gently 
  

   arched 
  bone, 
  of 
  almost 
  uniform 
  depth, 
  except 
  in 
  its 
  anterior 
  third, 
  

   which 
  gradually 
  contracts 
  and 
  ends 
  in 
  a 
  stout 
  inwardly-directed 
  

   process 
  ; 
  the 
  oral 
  border 
  is 
  convex, 
  and 
  the 
  hinder 
  two-thirds 
  of 
  the 
  

  

  1 
  See 
  figures 
  and 
  description 
  in 
  Proc. 
  Zool. 
  Soc. 
  1890, 
  pp. 
  346-350, 
  

   pis. 
  xxviii., 
  xxix. 
  

  

  