﻿192 
  

  

  ACTINOPTERYGII. 
  

  

  to 
  some 
  extent 
  the 
  superior 
  border 
  of 
  the 
  orbital 
  cavity 
  ; 
  this 
  in 
  

   our 
  nomenclature 
  is 
  evidently 
  the 
  squamosal 
  (sq.). 
  In 
  the 
  majority 
  

   of 
  Pycnodonts, 
  the 
  space 
  between 
  the 
  supraoccipital, 
  frontal, 
  and 
  

   squamosal 
  elements 
  is 
  occupied 
  on 
  each 
  side 
  by 
  a 
  single 
  parietal 
  

  

  Pi^. 
  30. 
  

  

  Mesturus 
  leedsi; 
  diagram 
  of 
  bones 
  ot 
  cranial 
  roof, 
  superior 
  aspect, 
  from 
  a 
  

  

  specimen 
  in 
  [the 
  collection 
  of 
  Alfred 
  N. 
  Leeds, 
  Esq., 
  Eyebury. 
  — 
  Oxford 
  

  

  Clay 
  ; 
  Peterborough. 
  

   eth., 
  ethmoidal 
  plates 
  ; 
  fr., 
  frontal 
  ; 
  m.eth., 
  median 
  ethmoidal 
  plate 
  ; 
  orb., 
  

  

  orbit; 
  sq., 
  squamosal 
  ; 
  s.occ, 
  supraoccipital; 
  s.t., 
  supratemporal 
  ; 
  opiates 
  

  

  in 
  parietal 
  region. 
  

  

  plate 
  ; 
  while 
  this 
  plate 
  m'Mesodon, 
  Microdon, 
  Stemmatodus, 
  Coelodus, 
  

   Palceobcdistum, 
  and 
  Pycnodus 
  exhibits 
  a 
  large 
  posteriorly-directed 
  

   process 
  with 
  digitate 
  extremity, 
  arising 
  about 
  the 
  middle 
  of 
  its 
  hinder 
  

   margin 
  and 
  extending 
  immediately 
  beneath 
  the 
  scales 
  of 
  the 
  trunk. 
  

   In 
  Mesturus 
  leedsi, 
  however, 
  the 
  parietal 
  region 
  is 
  occupied 
  by 
  

   two 
  or 
  more 
  irregular 
  plates 
  on 
  each 
  side 
  (x) 
  ; 
  and 
  neither 
  in 
  this 
  

   genus 
  nor 
  in 
  Gyrodus 
  is 
  there 
  any 
  posteriorly-directed 
  process. 
  The 
  

   ethmoidal 
  plates, 
  as 
  shown 
  in 
  Mesturus 
  leedsi, 
  are 
  most 
  remark- 
  

   able 
  (eth.). 
  The 
  region 
  in 
  advance 
  of 
  the 
  frontals 
  is 
  covered 
  with 
  

   polygonal 
  tesserge, 
  which 
  extend 
  even 
  over 
  the 
  smooth 
  premaxillae 
  ; 
  

  

  