﻿14 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol. 
  6T 
  

  

  united 
  with 
  the 
  exoccipital 
  along 
  its 
  external 
  border. 
  To 
  the 
  inside 
  

   of 
  the 
  postglenoid 
  process 
  and 
  at 
  the 
  origin 
  of 
  the 
  groove 
  for 
  the 
  

   external 
  auditory 
  meatus, 
  the 
  periotic 
  was 
  attached 
  to 
  the 
  skull. 
  A 
  

   thin 
  plate-like 
  process 
  of 
  the 
  squamosal 
  which 
  is 
  directed 
  inward 
  and 
  

   downward 
  contributes 
  the 
  external 
  wall 
  of 
  the 
  tympano-periotic 
  fossa; 
  

   the 
  internal 
  wall 
  is 
  formed 
  by 
  the 
  falcate 
  process 
  of 
  the 
  basioccipital. 
  

   Between 
  the 
  glenoid 
  process 
  and 
  the 
  internal 
  margin 
  of 
  the 
  glenoid 
  

   fossa 
  there 
  is 
  a 
  sharply 
  defined 
  longitudinal 
  depression, 
  which 
  curves 
  

   forward 
  from 
  the 
  base 
  of 
  the 
  groove 
  for 
  the 
  external 
  auditory 
  meatus 
  

   to 
  the 
  anterior 
  margin 
  of 
  the 
  squamosal. 
  The 
  contour 
  of 
  the 
  ventral 
  

   surface 
  of 
  the 
  paroccipital 
  process 
  resembles 
  that 
  of 
  Inia 
  and 
  Pla- 
  

   tanista. 
  There 
  are 
  some 
  minor 
  modifications 
  which 
  are 
  not 
  developed 
  

   on 
  the 
  paroccipital 
  processes 
  in 
  the 
  last-mentioned 
  genera 
  and 
  of 
  these 
  

   the 
  thin 
  crest 
  on 
  the 
  anterior 
  border 
  is 
  the 
  most 
  conspicuous. 
  

  

  Further 
  comparisons 
  with 
  skulls 
  of 
  the 
  living 
  Iniidae 
  and 
  with 
  

   Platanista 
  show 
  some 
  interesting 
  structural 
  modifications 
  in 
  the 
  

   region 
  of 
  the 
  orbitosphenoid. 
  In 
  Inia, 
  the 
  sphenoidal 
  fissure 
  is 
  rela- 
  

   tively 
  large, 
  bell-shaped 
  in 
  contour, 
  and 
  bounded 
  laterally 
  and 
  on 
  

   the 
  rear 
  by 
  the 
  basisphenoid, 
  and 
  anteriorly 
  by 
  the 
  orbitosphenoid. 
  

   On 
  their 
  outward 
  course, 
  the 
  trochlearis 
  and 
  the 
  ophthalmic 
  division 
  

   of 
  the 
  trigeminal 
  nerve 
  follow 
  the 
  groove 
  on 
  the 
  ventral 
  surface 
  of 
  

   an 
  attenuate 
  process 
  of 
  the 
  orbitosphenoid. 
  This 
  groove 
  is 
  rather 
  

   deep, 
  with 
  well-marked 
  lateral 
  walls; 
  the 
  process 
  as 
  a 
  whole 
  is 
  directed 
  

   obliquely 
  outward 
  and 
  forward, 
  terminating 
  near 
  the 
  base 
  of 
  the 
  

   broad 
  channel 
  on 
  the 
  supraorbital 
  process 
  of 
  the 
  frontal. 
  The 
  most 
  

   remarkable 
  peculiarity 
  of 
  the 
  orbitosphenoid 
  of 
  Inia 
  is 
  the 
  position 
  

   of 
  the 
  ectal 
  orifice 
  of 
  the 
  optic 
  canal. 
  This 
  orifice 
  appears 
  on 
  the 
  

   inner 
  wall 
  and 
  about 
  halfway 
  between 
  the 
  anterior 
  and 
  posterior 
  

   ends 
  of 
  the 
  groove 
  leading 
  forward 
  from 
  the 
  sphenoidal 
  fissure. 
  

   From 
  this 
  point 
  forward 
  all 
  three 
  nerves 
  occupy 
  the 
  same 
  groove. 
  

   Within 
  the 
  cranial 
  cavity, 
  the 
  orifice 
  of 
  each 
  optic 
  canal 
  is 
  placed 
  

   near 
  the 
  inner 
  angle 
  of 
  the 
  sphenoidal 
  fissure 
  and 
  is 
  overhung 
  by 
  a 
  

   thin 
  platelike 
  process 
  of 
  the 
  orbitosphenoid. 
  The 
  course 
  of 
  the 
  optic 
  

   canal 
  through 
  the 
  orbitosphenoid 
  parallels 
  the 
  groove 
  leading 
  forward 
  

   from 
  the 
  sphenoidal 
  fissure. 
  The 
  apex 
  of 
  the 
  process 
  of 
  the 
  orbito- 
  

   sphenoid 
  which 
  contributes 
  the 
  surface 
  of 
  the 
  groove 
  for 
  these 
  nerves 
  

   is 
  jagged, 
  and 
  projects 
  outward 
  and 
  forward 
  at 
  a 
  lower 
  level 
  than 
  the 
  

   ventral 
  surface 
  of 
  the 
  supraorbital 
  process. 
  

  

  In 
  Lipotes, 
  the 
  sphenoidal 
  fissure 
  is 
  narrower, 
  bounded 
  on 
  the 
  

   rear 
  and 
  on 
  the 
  outside 
  by 
  the 
  alisphenoid, 
  and 
  on 
  the 
  inside 
  and 
  

   in 
  front 
  by 
  the 
  orbitosphenoid. 
  The 
  orbitosphenoid 
  of 
  Lipotes 
  has 
  

   the 
  same 
  general 
  shape 
  as 
  that 
  of 
  Inia 
  and 
  is 
  likewise 
  characterized 
  

   by 
  a 
  narrow 
  attenuate 
  process 
  which 
  is 
  grooved 
  for 
  the 
  passage 
  of 
  

   nerves. 
  The 
  course 
  of 
  the 
  optic 
  nerve 
  is 
  slightly 
  modified 
  in 
  com- 
  

   parison 
  with 
  Inia. 
  This 
  nerve 
  does 
  not 
  pierce 
  the 
  orbitosphenoid, 
  

  

  