﻿A 
  STUDY 
  IX 
  CARCINOLOGT. 
  51 
  

  

  Each 
  of 
  the 
  lateral 
  wings 
  is 
  armed 
  anteriorly 
  with 
  n 
  prominenl 
  .-pine 
  ; 
  its 
  

  

  outer 
  bonier 
  is 
  nearly 
  .straight 
  and 
  makes 
  nearly 
  a 
  right 
  angle 
  with 
  tie 
  

   transverse 
  ledge 
  formed 
  by 
  the 
  median 
  lobes; 
  its 
  inner 
  border 
  is 
  thickened 
  

   and 
  curves 
  round 
  the 
  outer 
  extremity 
  of 
  the 
  mandibular 
  sternum. 
  The 
  last- 
  

   named 
  structure 
  is 
  a 
  projecting 
  curved 
  bar 
  intimately 
  fused 
  to 
  the 
  median 
  

   area 
  of 
  the 
  antennary 
  sternum, 
  but 
  marked 
  off 
  by 
  a 
  groove 
  which 
  in 
  the 
  

   middle 
  line 
  deepens 
  to 
  form 
  a 
  triangular 
  fossa. 
  Laterally 
  the 
  mandibular 
  

   sternum 
  seems 
  to 
  terminate 
  in 
  a 
  pair 
  of 
  incurved 
  prominences 
  for 
  articula- 
  

   tion 
  with 
  the 
  inner 
  articular 
  processes 
  of 
  the 
  mandibles, 
  but 
  is 
  clearly 
  

   prolonged 
  beyond 
  these 
  as 
  a 
  pair 
  of 
  diverging 
  horns 
  intimately 
  fused 
  to 
  the 
  

   postero-internal 
  edges 
  of 
  the 
  aliform 
  external 
  areas 
  of 
  the 
  an 
  tennary 
  sternum 
  

   but 
  projecting 
  slightly 
  beyond 
  the 
  hitter. 
  The 
  mandibular 
  epimera 
  are 
  

   largely 
  membranous, 
  but 
  their 
  antero-internal 
  ends 
  are 
  calcified 
  to 
  form 
  the 
  

   two 
  plates 
  marked 
  in 
  fig. 
  26 
  which 
  form 
  the 
  roof 
  of 
  the 
  anterior 
  part 
  of 
  

   the 
  exhalant 
  branchial 
  canals. 
  A 
  comparison 
  of 
  figs. 
  19, 
  22, 
  and 
  27 
  will 
  I 
  

   think, 
  convince 
  the 
  reader 
  that 
  in 
  the 
  whole 
  make-up 
  of 
  the 
  pre-oral 
  region 
  

   the 
  Dromiacea 
  have 
  departed 
  further 
  from 
  the 
  Astacnran 
  type 
  than 
  have 
  the 
  

   Raninidse, 
  particularly 
  in 
  the 
  reduction 
  of 
  the 
  rostrum, 
  the 
  greater 
  develop- 
  

   ment 
  of 
  the 
  suborbital 
  lobe 
  of 
  the 
  carapace, 
  the 
  membranous 
  condition 
  of 
  

   the 
  inner 
  moieties 
  of 
  the 
  ocular 
  peduncles, 
  the 
  size 
  ami 
  relative 
  positions 
  

   of 
  the 
  antennulary 
  and 
  antennary 
  fossae. 
  Further, 
  the 
  modifications 
  of 
  the 
  

   antennary 
  and 
  mandibular 
  sterna 
  are 
  widely 
  divergent 
  in 
  the 
  two 
  groups. 
  

   It 
  may 
  be 
  objected, 
  and 
  there 
  would 
  be 
  some 
  force 
  in 
  the 
  objection, 
  

   that 
  the 
  genus 
  Dromia, 
  on 
  which 
  I 
  have 
  relied 
  for 
  study 
  of 
  details, 
  is 
  a 
  

   much 
  modified 
  and 
  specialised 
  genus 
  of 
  the 
  Dromiacea, 
  and 
  that 
  I 
  should 
  have 
  

   directed 
  my 
  attention 
  rather 
  to 
  the 
  more 
  primitive 
  genera, 
  Homolodromia 
  and 
  

   Dicranodrornia, 
  on 
  which 
  Bouvier 
  so 
  largely 
  relies 
  in 
  establishing 
  his 
  theory 
  

   of 
  the 
  Astacuran 
  origin 
  of 
  crabs. 
  Unfortunately 
  examples 
  of 
  these 
  rare 
  

   and 
  instructive 
  forms 
  were 
  not 
  at 
  my 
  disposal, 
  but 
  they 
  have 
  been 
  described 
  

   in 
  sufficient 
  detail 
  by 
  A. 
  Milne 
  Edwards 
  and 
  Bouvier 
  (28), 
  and 
  a 
  reference 
  

   to 
  pi. 
  i. 
  fig. 
  2 
  and 
  pi. 
  iii. 
  fig. 
  2 
  of 
  their 
  admirable 
  memoir 
  will 
  convince 
  the 
  

   reader 
  that 
  I 
  am 
  justified 
  in 
  extending 
  the 
  results 
  of 
  my 
  study 
  oi' 
  the 
  genus 
  

   Dromia 
  to 
  the 
  more 
  primitive 
  members 
  of 
  the 
  group 
  and 
  in 
  asserting 
  that 
  in 
  

   the 
  make-up 
  of 
  the 
  pre-oral 
  region, 
  as 
  in 
  other 
  characters 
  previously 
  ceall 
  

   with, 
  tlte 
  Dormiacea 
  have 
  departed 
  more 
  widely 
  from 
  the 
  Astacuran 
  type 
  

   than 
  have 
  the 
  Raninidaj, 
  and 
  therefore 
  cannot 
  be 
  enrolled 
  in 
  the 
  ancestry 
  of 
  

   the 
  latter. 
  

  

  . 
  I 
  submit 
  that, 
  by 
  a 
  detailed 
  study 
  and 
  comparison 
  of 
  the 
  nervous 
  system, 
  

   the 
  endophragmal 
  system 
  of 
  the 
  thorax, 
  and 
  the 
  cephalic 
  segments, 
  I 
  have 
  

   proved 
  the 
  first 
  part 
  of 
  my 
  thesis 
  and 
  have 
  shown 
  that, 
  whilsl 
  there 
  is 
  plenty 
  

   of 
  evidence 
  for 
  deriving 
  each 
  group 
  separately 
  from 
  an 
  Astacuran 
  ancestor 
  

   the 
  Raninidse 
  cannot 
  be 
  directly 
  descended 
  from 
  a 
  Dromiacean 
  stock. 
  The 
  

  

  ±* 
  

  

  