﻿160 
  Mr. 
  A. 
  Sanders 
  on 
  the 
  

  

  termed 
  the 
  " 
  Falx 
  " 
  by 
  Prof. 
  Burt. 
  G. 
  Wilder 
  *, 
  who 
  was 
  not 
  

   certain 
  that 
  it 
  was 
  continuous 
  with 
  the 
  lower 
  surface 
  of 
  the 
  

   tela 
  choroidea. 
  The 
  ventricular 
  partition 
  is 
  thus 
  seen 
  to 
  be 
  

   made 
  up 
  for 
  a 
  great 
  part 
  of 
  its 
  extent 
  by 
  tissue 
  which 
  is 
  not 
  

   nervous. 
  The 
  function 
  of 
  these 
  two 
  bodies 
  is 
  probably, 
  as 
  

   Wiedersheim 
  f 
  suggests 
  in 
  Protopterus, 
  also 
  in 
  this 
  case, 
  for 
  

   the 
  purpose 
  of 
  secreting 
  the 
  cerebro-spinal 
  fluid. 
  

  

  Over 
  the 
  third 
  ventricle 
  the 
  pia 
  mater 
  forms 
  a 
  sort 
  of 
  

   conical 
  membranous 
  cap, 
  in 
  the 
  summit 
  of 
  which 
  is 
  found 
  a 
  

   body, 
  the 
  pineal 
  gland 
  ; 
  but 
  this 
  shows 
  no 
  retinal 
  structure, 
  

   even 
  in 
  the 
  modified 
  form 
  described 
  by 
  Ahlborn 
  in 
  Petro- 
  

   myzon, 
  much 
  less 
  the 
  comparatively 
  high 
  development 
  

   described 
  by 
  Spencer 
  J 
  in 
  Lacertilia 
  and 
  by 
  Graaf 
  § 
  in 
  Anguis 
  

   fragilis. 
  This 
  cover 
  resembles 
  that 
  in 
  Protopterus 
  \\, 
  but 
  it 
  

   extends 
  much 
  higher 
  in 
  Ceratodus 
  ; 
  in 
  Lepidosteus 
  also 
  

   a 
  membrane 
  resembling 
  this 
  is 
  described 
  by 
  Balfour 
  and 
  

   Parker 
  % 
  but 
  this 
  latter 
  appears 
  to 
  resemble 
  more 
  Protopterus 
  

   than 
  Ceratodus 
  in 
  appearance. 
  In 
  Rhina 
  squatina 
  and 
  Acan- 
  

   thias 
  vulgaris 
  also 
  there 
  is 
  a 
  membranous 
  cover 
  to 
  the 
  thala- 
  

   mencephalon, 
  which 
  seems 
  to 
  be 
  a 
  homologous 
  structure. 
  

  

  The 
  pia 
  mater 
  is 
  more 
  closely 
  attached 
  to 
  the 
  rhinencepha- 
  

   Ion 
  than 
  to 
  the 
  remainder 
  of 
  the 
  brain, 
  where 
  it 
  can 
  be 
  easily 
  

   stripped 
  off 
  with 
  a 
  pair 
  of 
  forceps. 
  On 
  the 
  ventral 
  surface 
  

   (PL 
  XI. 
  fig. 
  15) 
  of 
  the 
  medulla 
  oblongata 
  and 
  the 
  commencing 
  

   spinal 
  cord, 
  immediately 
  in 
  front 
  of 
  the 
  entrance 
  of 
  the 
  ante- 
  

   rior 
  root 
  of 
  the 
  trifacial 
  nerve, 
  the 
  anterior 
  median 
  fissure 
  is 
  

   filled 
  up 
  with 
  a 
  development 
  of 
  the 
  pia 
  mater 
  of 
  a 
  semicarti- 
  

   laginous 
  consistence 
  ; 
  it 
  is 
  not, 
  however, 
  by 
  any 
  means 
  of 
  the 
  

   structure 
  of 
  cartilage, 
  but 
  consists 
  of 
  thick, 
  round, 
  smooth 
  

   fibres, 
  several 
  of 
  which 
  join 
  together 
  into 
  one 
  at 
  the 
  external 
  

   edge 
  in 
  a 
  transverse 
  direction, 
  forming 
  a 
  sort 
  of 
  fenestrated 
  

   structure, 
  whereby 
  it 
  becomes 
  thicker 
  in 
  the 
  central 
  line 
  and 
  

   thins 
  off 
  exteriorly, 
  where 
  it 
  joins 
  the 
  ordinary 
  membrane 
  ; 
  it 
  

   appears 
  to 
  act 
  as 
  an 
  elastic 
  pad 
  which 
  would 
  protect 
  the 
  me- 
  

   dulla 
  oblongata 
  from 
  sudden 
  shock 
  ; 
  but 
  why 
  the 
  other 
  parts 
  

   of 
  the 
  brain 
  should 
  not 
  be 
  as 
  well 
  protected 
  does 
  not 
  appear. 
  

  

  Like 
  Protopterus, 
  Ceratodus 
  presents 
  a 
  wide 
  and 
  well- 
  

   developed 
  medulla 
  oblongata 
  connected 
  by 
  a 
  narrow 
  mesen- 
  

   cephalon 
  and 
  thalamencephalon 
  with 
  the 
  prosencephalon 
  ; 
  but 
  

   there 
  is 
  no 
  sudden 
  angle 
  between 
  the 
  former 
  and 
  the 
  latter 
  

   as 
  is 
  described 
  by 
  Wiedersheim 
  ** 
  in 
  Protopterus. 
  

  

  * 
  L. 
  c. 
  p. 
  546. 
  t 
  L. 
  c. 
  p. 
  71. 
  

  

  I 
  Quart. 
  Journ. 
  Micr. 
  Sci. 
  1886. 
  

  

  § 
  Zool. 
  Auzeiger, 
  March 
  1886. 
  

  

  || 
  Wiedersheim, 
  /. 
  c. 
  p. 
  71, 
  fig. 
  21. 
  

  

  If 
  Phil. 
  Trans. 
  1882. 
  ** 
  L. 
  c. 
  p. 
  70. 
  

  

  