﻿38 
  DR. 
  G. 
  C. 
  BOURNE 
  ON 
  THE 
  RANINID^E 
  : 
  

  

  epimeral 
  walls 
  in 
  this 
  area, 
  being 
  exposed, 
  are 
  strongly 
  calcined. 
  This 
  

   exposure 
  of 
  a 
  large 
  portion 
  of 
  the 
  epimeral 
  surface 
  is 
  a 
  remarkable 
  and 
  

   unique 
  feature 
  in 
  the 
  Raninidse. 
  Even 
  in 
  the 
  Astacura, 
  where 
  the 
  lower 
  

   edge 
  of 
  the 
  branchiostegite 
  is 
  free, 
  it 
  reaches 
  down 
  to 
  the 
  bases 
  of 
  the 
  limbs, 
  

   and 
  in 
  all 
  crabs 
  the 
  lower 
  edge 
  of 
  the 
  branchiostegite 
  is 
  so 
  closely 
  adapted 
  

   to 
  the 
  coxa? 
  of 
  the 
  thoracic 
  limbs 
  as 
  to 
  leave 
  no 
  passage 
  for 
  water 
  into 
  the 
  

   branchial 
  chamber 
  behind 
  the 
  chelae. 
  This 
  peculiarity 
  did 
  not 
  escape 
  the 
  

   attention 
  of 
  Milne 
  Edwards, 
  but 
  subsequent 
  authors 
  seem 
  to 
  have 
  paid 
  very 
  

   little 
  attention 
  to 
  it. 
  If 
  we 
  now 
  examine 
  the 
  interior 
  of 
  the 
  thorax, 
  we 
  find 
  

   that 
  the 
  apodemes 
  between 
  the 
  ventral 
  and 
  exposed 
  parts 
  of 
  the 
  three 
  

   thoracic 
  epimera 
  in 
  question 
  are 
  shallow 
  and 
  form 
  nothing 
  more 
  than 
  two 
  

   inconspicuous 
  thickenings 
  internally, 
  strengthening 
  the 
  framework 
  of 
  this 
  

   part 
  of 
  the 
  flanks 
  but 
  not 
  projecting 
  into 
  the 
  cavity 
  of 
  the 
  body. 
  But 
  the 
  

   apodemes 
  above 
  the 
  line 
  of 
  the 
  adherence 
  of 
  the 
  branchiostegites 
  are 
  very 
  

   deep 
  and 
  form 
  well-marked 
  endopleurites 
  which, 
  as 
  usual, 
  bifurcate 
  and 
  their 
  

   branches 
  unite 
  with 
  the 
  expanded 
  summits 
  of 
  the 
  endosternites 
  next 
  in 
  front 
  

   and 
  next 
  behind 
  them. 
  It 
  appears 
  that 
  the 
  excessive 
  depth 
  of 
  the 
  sternal 
  

   canal 
  is 
  correlated 
  with 
  the 
  restriction 
  of 
  the 
  endopleurites 
  to 
  the 
  upper 
  

   moieties 
  of 
  the 
  epimera 
  or, 
  to 
  put 
  it 
  in 
  another 
  way, 
  with 
  the 
  downgrowth 
  of 
  

   the 
  ventral 
  edges 
  of 
  the 
  epimera 
  beyond 
  the 
  line 
  of 
  the 
  attachment 
  of 
  the 
  

   branchiostegite. 
  The 
  ventral 
  extension 
  of 
  the 
  epimera 
  is 
  an 
  adaptive 
  

   feature, 
  and 
  receives 
  its 
  explanation 
  when 
  the 
  habits 
  of 
  the 
  animals 
  are 
  taken 
  

   into 
  account. 
  The 
  Raninidoe 
  are 
  digging 
  crabs, 
  with 
  large 
  and 
  powerful 
  

   chelipeds 
  and 
  first, 
  second, 
  and 
  third 
  pereiopods. 
  In 
  many 
  the 
  fourth 
  pair 
  

   of 
  pereiopods 
  is 
  weak 
  and 
  slender, 
  though 
  in 
  Raniua 
  this 
  last 
  pair 
  is 
  not 
  

   much 
  weaker 
  than 
  the 
  others. 
  The 
  powerful 
  digging 
  limbs 
  imply 
  powerful 
  

   abductor 
  muscles 
  and 
  large 
  adductor 
  muscle-cavities. 
  Reference 
  to 
  fig. 
  9 
  

   shows 
  that 
  these 
  cavities 
  are 
  in 
  fact 
  unusually 
  deep, 
  and 
  their 
  extent 
  is 
  due 
  

   to 
  the 
  great 
  length 
  of 
  the 
  endosternites 
  within 
  the 
  body 
  and 
  to 
  the 
  large 
  

   ventral 
  extensions 
  of 
  the 
  epimera 
  forming 
  their 
  outer 
  walls. 
  The 
  same 
  

   figure 
  shows 
  that, 
  as 
  in 
  crabs, 
  the 
  abductor 
  muscle-cavities 
  lie 
  above 
  and 
  not 
  

   alongside 
  of 
  the 
  adductor 
  muscle-cavities. 
  But 
  cancroid 
  as 
  this 
  character 
  

   is, 
  it 
  has 
  clearly 
  a 
  very 
  different 
  origin 
  from 
  the 
  similar 
  arrangement 
  

   in 
  crabs. 
  

  

  To 
  turn 
  to 
  another 
  feature 
  noted 
  by 
  de 
  Haan, 
  " 
  thoracis 
  interior 
  offert 
  

   sollam 
  turcicam 
  uti 
  tantum 
  in 
  Brachyuris." 
  What 
  is 
  this 
  sella 
  turcica 
  ? 
  

   The 
  name 
  is 
  due 
  to 
  Audouin 
  and 
  Edwards, 
  and 
  the 
  latter 
  author 
  has 
  

   described 
  the 
  elements 
  entering 
  into 
  its 
  composition 
  with 
  great 
  minuteness 
  

   on 
  p. 
  34 
  of 
  the 
  first 
  volume 
  of 
  the 
  ' 
  Histoire 
  Naturelle 
  des 
  (!rustaces.' 
  

   Paraphrasing 
  his 
  description, 
  the 
  sella 
  turcica 
  posterior 
  is 
  a 
  little 
  arch 
  or 
  

   vault, 
  concave 
  from 
  side 
  to 
  side 
  and 
  convex 
  from 
  front 
  to 
  back, 
  formed 
  by 
  

   the 
  forward 
  extension 
  and 
  fusion 
  together 
  in 
  the 
  middle 
  line 
  of 
  the 
  nearly 
  

  

  