﻿72 
  DTI. 
  G. 
  C, 
  BOUENE 
  ON 
  THE 
  RANINID^ 
  : 
  

  

  the 
  second 
  and 
  first 
  maxillipeds. 
  It 
  is 
  to 
  direct 
  this 
  current 
  under 
  the 
  edge 
  

   of 
  the 
  mouth 
  frame 
  into 
  the 
  branchial 
  chamber 
  that 
  the 
  exopod 
  of 
  the 
  second 
  

   maxilliped 
  is 
  elongated 
  and 
  modified 
  by 
  being 
  channelled 
  along 
  its 
  ventral 
  

   surface. 
  The 
  modification 
  is 
  carried 
  furthest 
  in 
  Notosceles 
  (tig. 
  44) 
  and 
  

   Rcuiina 
  (fig. 
  42), 
  two 
  forms 
  in 
  which 
  the 
  accessory 
  antennary 
  modifications 
  

   are 
  highly 
  developed, 
  and 
  in 
  these 
  the 
  exopod 
  has 
  lost 
  its 
  flagellum. 
  But 
  in 
  

   Notopus, 
  in 
  which 
  genus 
  the 
  antennas 
  are 
  most 
  highly 
  specialised 
  for 
  respira- 
  

   tory 
  purposes, 
  the 
  exopod 
  in 
  question 
  (fig. 
  32) 
  retains 
  a 
  large 
  flagellum, 
  

   and 
  the 
  flagellum 
  is 
  present 
  but 
  small 
  in 
  Lyreidus 
  (fig. 
  52) 
  and 
  Raninoides 
  

   (fig. 
  49). 
  This 
  may 
  seem 
  a 
  trivial 
  character, 
  but 
  it 
  gains 
  importance 
  when 
  

   comparison 
  is 
  made 
  with 
  the 
  same 
  appendage 
  in 
  the 
  Leucosiida?. 
  In 
  Philyra 
  

   Icevis 
  for 
  example 
  (tig. 
  54) 
  the 
  exopod 
  of 
  the 
  second 
  maxilliped 
  is 
  of 
  simple 
  

   shape, 
  shorter 
  than 
  the 
  endopod 
  and 
  convex 
  ventrally. 
  In 
  this 
  species, 
  as 
  in 
  

   all 
  the 
  Leucosiida?, 
  there 
  is 
  a 
  definite 
  exostegal 
  canal 
  covered 
  in 
  below 
  by 
  the 
  

   exopod 
  of 
  the 
  maxilliped 
  : 
  the 
  second 
  maxilliped, 
  therefore, 
  does 
  not 
  lie 
  in 
  

   the 
  course 
  of 
  the 
  incurrent 
  stream 
  and 
  is 
  not 
  modified. 
  I 
  have 
  already 
  

   (p. 
  54) 
  called 
  attention 
  to 
  the 
  difference 
  between 
  the 
  first 
  maxillipeds 
  of 
  

   the 
  Raninida? 
  and 
  those 
  of 
  other 
  Oxystomatous 
  crabs. 
  Indeed, 
  as 
  I 
  have 
  

   already 
  indicated 
  when 
  dealing 
  with 
  the 
  endophragmal 
  skeleton, 
  the 
  anterior 
  

   thoracic 
  sterna, 
  and 
  other 
  structures, 
  the 
  Leucosiida? 
  differ 
  in 
  a 
  hundred 
  ways 
  

   from 
  the 
  Raninida?, 
  and 
  I 
  regard 
  the 
  comparison 
  with 
  Lyreidus 
  as 
  wholly 
  

   illusory. 
  The 
  exostegal 
  canal 
  of 
  the 
  Leucosiida? 
  is 
  most 
  probably 
  derived 
  

   from 
  the 
  shallow 
  groove 
  in 
  the 
  pterygostome 
  occupying 
  the 
  same 
  position 
  

   in 
  Cyclodorippe. 
  It 
  would 
  be 
  interesting 
  and 
  not 
  wholly 
  irrelevant 
  to 
  

   attempt 
  to 
  trace 
  out 
  the 
  various 
  lines 
  of 
  descent 
  in 
  the 
  families 
  of 
  Oxysto- 
  

   mata 
  other 
  than 
  the 
  Raninida?, 
  and 
  I 
  have 
  collected 
  much 
  evidence 
  on 
  this 
  

   subject 
  based 
  on 
  a 
  study 
  of 
  the 
  endophragmal 
  skeleton 
  and 
  other 
  anatomical 
  

   features 
  not 
  taken 
  into 
  account 
  by 
  M. 
  Edwards 
  and 
  Bouvier. 
  But 
  it 
  would 
  

   almost 
  double 
  the 
  length 
  of 
  this 
  memoir 
  if 
  I 
  were 
  to 
  make 
  the 
  attempt. 
  

   I 
  have 
  already 
  written 
  enough, 
  and 
  have, 
  I 
  hope, 
  proved 
  my 
  main 
  thesis 
  that 
  

   the 
  Raninida? 
  cannot 
  have 
  descended, 
  by 
  way 
  of 
  the 
  Dorippida? 
  and 
  the 
  

   Dromiacen, 
  from 
  the 
  Macrura. 
  They 
  have 
  originated 
  independently 
  from 
  

   the 
  last 
  named, 
  and 
  must 
  therefore 
  be 
  separated 
  from 
  other 
  OxystOmes 
  as 
  a 
  

   separate 
  tribe 
  Gymnopleura. 
  I 
  have 
  endeavoured 
  to 
  show 
  that 
  the 
  numerous 
  

   brachyuran 
  features 
  exhibited 
  by 
  this 
  tribe 
  must 
  be 
  explained 
  by 
  their 
  having 
  

   adopted 
  the 
  same 
  habits 
  as 
  other 
  crabs, 
  and 
  that 
  such 
  modifications 
  as 
  the 
  

   redaction 
  of 
  the 
  abdomen 
  ; 
  the 
  loss 
  of 
  the 
  uropods 
  ; 
  the 
  adherence 
  of 
  the 
  

   branchiostegite 
  to 
  the 
  thoracic 
  epimera 
  (exhibited, 
  however, 
  in 
  a 
  quite 
  

   peculiar 
  manner 
  in 
  the 
  Gymnopleura) 
  ; 
  the 
  expansion 
  of 
  the 
  ischium 
  and 
  

   merus 
  of 
  the 
  third 
  maxillipeds; 
  the 
  'broadening 
  of 
  the 
  front, 
  with 
  which 
  

   is 
  correlated 
  the 
  formation 
  of 
  orbital 
  cavities 
  and 
  the 
  reduction 
  of 
  the 
  

   lost 
  rum 
  ; 
  the 
  great 
  development 
  of 
  the 
  muscle-cavities 
  of 
  the 
  thorax, 
  — 
  are 
  all 
  

   characters 
  of 
  high 
  survival 
  value 
  in 
  decapods 
  which 
  have 
  acquired 
  the 
  habit 
  

  

  