﻿[53] 
  DEVELOPMENT 
  OF 
  OSSEOUS 
  FISHES. 
  541 
  

  

  superimposed 
  outlines 
  of 
  the 
  cartilages 
  of 
  the 
  skull, 
  as 
  cut 
  at 
  successive 
  

   levels 
  iu 
  a 
  series 
  of 
  sections 
  of 
  uniform 
  thickness, 
  extending 
  from 
  the 
  

   outer 
  side 
  of 
  the 
  head 
  to 
  the 
  middle 
  line. 
  Stereograms 
  of 
  this 
  character 
  

   may 
  be 
  readily 
  constructed 
  with 
  the 
  aid 
  of 
  the 
  camera 
  lucida 
  from 
  a 
  se- 
  

   ries 
  of 
  sections, 
  if 
  a 
  uniform 
  amplification 
  is 
  employed 
  and 
  patience 
  and 
  

   care 
  is 
  exercised 
  in 
  drawing 
  the 
  outlines. 
  

  

  It 
  will 
  at 
  once 
  be 
  noticed 
  that, 
  as 
  compared 
  with 
  the 
  chondrocranium 
  

   of 
  Salmo, 
  as 
  figured 
  by 
  Parker, 
  52 
  or 
  of 
  Gambusia, 
  as 
  figured 
  by 
  my- 
  

   self, 
  53 
  the 
  cartilaginous 
  cranium 
  of 
  Ictalurus 
  presents 
  some 
  very 
  impor- 
  

   tant 
  modifications. 
  These 
  involve 
  mainly 
  the 
  structure 
  of 
  the 
  palatop- 
  

   terygoid 
  arch 
  PIPt, 
  which 
  is 
  composed 
  in 
  Ictalurus 
  of 
  two 
  pieces 
  in- 
  

   stead 
  of 
  one. 
  The 
  narrow 
  bar 
  T 
  Cr, 
  representing 
  the 
  tegmen 
  cranii 
  

   in 
  Ictalurus, 
  is 
  much 
  wider 
  in 
  both 
  the 
  other 
  types 
  named, 
  and 
  the 
  build 
  

   of 
  the 
  skull 
  in 
  the 
  type 
  here 
  described 
  is 
  complicated 
  by 
  the 
  presence 
  of 
  

   no 
  less 
  than 
  three 
  pairs 
  of 
  cartilaginous 
  appendages 
  for 
  the 
  support 
  of 
  

   the 
  barbels, 
  representing 
  chondrified 
  elements 
  which 
  are 
  probably 
  not 
  

   found 
  in 
  the 
  skulls 
  of 
  young 
  fishes 
  of 
  any 
  other 
  type. 
  The 
  skulls 
  of 
  

   the 
  Nematognathi 
  are 
  therefore 
  distinguishable 
  from 
  those 
  of 
  other 
  or- 
  

   dinal 
  groups 
  of 
  fishes 
  at 
  a 
  very 
  early 
  period 
  of 
  development. 
  

  

  The 
  chondrocranium 
  of 
  the 
  young 
  Ictalurus, 
  as 
  a 
  whUe, 
  is 
  depressed, 
  

   but 
  relatively 
  far 
  less 
  so 
  than 
  in 
  the 
  adult. 
  The 
  auditory 
  apparatus 
  is 
  

   quite 
  completely 
  covered 
  in 
  laterally 
  and 
  inferiorly 
  by 
  a 
  cartilaginous 
  

   investment 
  in 
  the 
  region 
  marked 
  Au. 
  Below 
  and 
  mesially, 
  the 
  para- 
  

   chordal 
  elements 
  Pa 
  C 
  are 
  found, 
  although 
  now 
  quite 
  completely 
  fused 
  

   with 
  the 
  audi 
  tory 
  capsules 
  laterally 
  and 
  the 
  trabecular 
  bars 
  Tr 
  anteriorly. 
  

   The 
  cartilaginous 
  brain 
  box 
  is 
  perforated 
  laterally 
  on 
  either 
  side 
  in 
  the 
  

   exoccipital 
  region 
  to 
  give 
  passage 
  to 
  the 
  ninth 
  and 
  tenth 
  pairs 
  of 
  nerves 
  

   at 
  IX 
  and 
  X. 
  In 
  advance 
  of 
  the 
  articulation 
  of 
  the 
  hyomandibular 
  bar, 
  

   Hm, 
  there 
  is 
  a 
  large 
  lateral 
  fenestra 
  in 
  the 
  cranial 
  box 
  through 
  which 
  

   the 
  second 
  or 
  optic, 
  the 
  fifth 
  or 
  trigeminal, 
  and 
  the 
  seventh 
  or 
  facial 
  

   nerves 
  pass 
  at 
  II, 
  \, 
  and 
  VII. 
  Just 
  in 
  front 
  of 
  this 
  is 
  the 
  orbit 
  0, 
  and 
  

   forming 
  its 
  lower 
  inner 
  walls 
  is 
  seen 
  the 
  chondrified 
  plate 
  Ps 
  destined 
  

   to 
  form 
  the 
  presphenoid. 
  In 
  front 
  of 
  the 
  orbit 
  there 
  is 
  a 
  high 
  ect- 
  

   ethmoidal 
  ridge, 
  EE, 
  in 
  advance 
  of 
  which 
  lies 
  the 
  olfactory 
  fossa 
  01. 
  

   Anteriorly 
  the 
  skull 
  terminates 
  in 
  the 
  trabecular 
  rostrum 
  B. 
  There 
  is 
  

   a 
  wide 
  fontanelle 
  behind 
  the 
  tegmen 
  cranii 
  or 
  frontal 
  bridge, 
  and 
  a 
  

   smaller 
  one 
  in 
  front 
  of 
  it. 
  These 
  fontanelles 
  persist 
  in 
  the 
  median 
  line 
  

   as 
  narrow 
  clefts, 
  partly 
  separating 
  the 
  frontals 
  in 
  the 
  ossified 
  cranium 
  

   of 
  the 
  adult. 
  At 
  the 
  posterior 
  end 
  of 
  the 
  cranial 
  box 
  and 
  in 
  the 
  median 
  

   line, 
  the 
  supraoccipital, 
  so, 
  is 
  developed 
  as 
  a 
  separate 
  block 
  of 
  cartilage. 
  

   The 
  extent 
  to 
  which 
  the 
  chorda 
  js 
  prolonged 
  iuto 
  the 
  base 
  of 
  the 
  skull 
  

   is 
  indicated 
  by 
  the 
  dotted 
  line 
  below 
  the 
  auditory 
  capsule 
  Au. 
  

  

  62 
  W. 
  K. 
  Parker 
  and 
  G. 
  T. 
  Bettany. 
  Morphology 
  of 
  the 
  skull, 
  London, 
  1877, 
  p. 
  59, 
  

   fig. 
  17. 
  

  

  " 
  J. 
  A. 
  Ryder. 
  Development 
  of 
  Viviparous 
  Oss. 
  Fishes. 
  Proc. 
  U. 
  S. 
  Nat. 
  Mus., 
  1885, 
  

   pi. 
  x, 
  pp. 
  150-151. 
  

  

  