﻿42 
  DR. 
  G. 
  C. 
  BOURNE 
  ON 
  THE 
  RANINID.E 
  : 
  

  

  be 
  distinguished 
  in 
  it. 
  Henee 
  there 
  is 
  nothing 
  to 
  correspond 
  with 
  the 
  sella 
  

   turcica 
  anterior 
  of 
  the 
  Astacura. 
  In 
  Dromia 
  vulgaris 
  much 
  the 
  same 
  elements 
  

   can 
  be 
  identified 
  as 
  in 
  the 
  Raninida?, 
  but 
  the 
  sternal 
  and 
  pleural 
  apodonies 
  

   of 
  intersegment 
  ix/x 
  do 
  not 
  meet 
  to 
  form 
  a 
  sternal 
  canal. 
  Similarly 
  the 
  

   sternal 
  and 
  pleural 
  apodemes 
  of 
  intersegment 
  viii/ix 
  unite 
  on 
  each 
  side 
  to- 
  

   form 
  a 
  curved 
  shelf 
  running 
  inwards 
  from 
  the 
  thoracic 
  wall, 
  but 
  leave 
  a 
  wide 
  

   space 
  open 
  in 
  the 
  middle 
  line. 
  The 
  pleural 
  apodemes 
  vi/vii 
  are 
  fairly 
  well 
  

   developed 
  and 
  project 
  some 
  way 
  inwards 
  and 
  backwards, 
  but 
  are 
  far 
  from 
  

   approximating 
  in 
  the 
  middle 
  line. 
  The 
  intersegment 
  v/vi 
  is 
  fairly 
  stout, 
  but 
  

   is 
  simply 
  an 
  arthrophragm 
  without 
  any 
  ingrowths 
  that 
  can 
  be 
  distinguished 
  

   as 
  endosternites, 
  and 
  though 
  the 
  sixth 
  pleura 
  are 
  well 
  developed, 
  they 
  have 
  

   no 
  inwardly 
  projecting 
  apodemes 
  separating 
  them 
  from 
  the 
  membrane 
  that 
  

   corresponds 
  with 
  the 
  fifth 
  pleuron. 
  In 
  all 
  these 
  respects 
  Dromia 
  has 
  

   departed 
  further 
  from 
  the 
  Astacuran 
  condition 
  than 
  have 
  the 
  Raninida?. 
  In 
  

   another 
  matter 
  also, 
  the 
  Raninidse 
  stand 
  closer 
  to 
  the 
  Astacura. 
  In 
  the 
  last 
  

   named, 
  for 
  example 
  in 
  Nephrops, 
  the 
  ninth, 
  eighth, 
  and 
  seventh 
  sterna 
  do 
  not 
  

   lie 
  in 
  the 
  same 
  straight 
  line 
  as 
  the 
  thoracic 
  sterna 
  behind 
  them, 
  but 
  are 
  

   directed 
  upwards, 
  so 
  that 
  a 
  line 
  drawn 
  through 
  them 
  forms 
  an 
  angle 
  of 
  about 
  

   140° 
  with 
  a 
  line 
  drawn 
  through 
  the 
  hinder 
  ends 
  of 
  the 
  more 
  posterior 
  

   thoracic 
  sterna. 
  

  

  In 
  the 
  Raninida?, 
  as 
  shown 
  in 
  fig. 
  17 
  for 
  Aotopus, 
  a 
  line 
  drawn 
  through 
  

   the 
  seventh, 
  eighth, 
  and 
  ninth 
  sterna 
  forms 
  an 
  angle 
  of 
  134° 
  with 
  a 
  line 
  

   drawn 
  through 
  the 
  tenth 
  and 
  eleventh 
  sterna. 
  In 
  Dromia 
  the 
  corresponding- 
  

   lines 
  meet 
  at 
  an 
  angle 
  of 
  108°. 
  This 
  is 
  another 
  character 
  in 
  which 
  the 
  

   Dromiacea 
  have 
  departed 
  further 
  from 
  the 
  Astacuran 
  type 
  than 
  the 
  Raninida?, 
  

   but, 
  as 
  regards 
  the 
  six 
  post-oral 
  segments, 
  it 
  would 
  not 
  be 
  quite 
  accurate 
  to 
  

   say 
  that 
  they 
  have 
  advanced 
  further 
  towards 
  the 
  cancroid 
  type. 
  There 
  are 
  

   certain 
  points 
  of 
  resemblance 
  between 
  the 
  Dromiacea 
  and 
  the 
  Raninidse 
  

   which 
  may 
  be 
  taken 
  as 
  evidence 
  of 
  but 
  slight 
  modification 
  from 
  the 
  common 
  

   Astacuran 
  stock 
  from 
  which 
  both 
  have 
  descended 
  : 
  not, 
  however, 
  as 
  evidence 
  

   that 
  one 
  group 
  has 
  descended 
  from 
  the 
  other. 
  The 
  evidence 
  so 
  far 
  tendered 
  

   shows 
  that 
  the 
  Raninidse 
  in 
  their 
  nervous 
  system 
  and 
  in 
  the 
  characters 
  of 
  the 
  

   endophragmal 
  skeleton 
  are 
  much 
  more 
  nearly 
  akin 
  to 
  the 
  Astacura 
  than 
  are 
  

   Dromiacea 
  ; 
  they 
  are, 
  therefore, 
  the 
  more 
  primitive 
  group 
  and 
  cannot 
  have 
  

   descended 
  from 
  the 
  less 
  primitive. 
  In 
  both 
  groups 
  the 
  departures 
  from 
  the 
  

   Macruran 
  type 
  as 
  exhibited 
  in 
  the 
  first 
  six 
  post-oral 
  segments 
  are 
  due 
  to 
  the 
  

   acquisition 
  of 
  a 
  broad 
  sternum 
  in 
  the 
  tenth 
  segment 
  and 
  the 
  upward 
  cant 
  of 
  

   the 
  sterna 
  immediately 
  in 
  front 
  of 
  it, 
  with 
  which 
  are 
  associated 
  the 
  retreat 
  

   of 
  the 
  sub-cesophageal 
  ganglion 
  mass 
  into 
  the 
  thorax 
  (see 
  fig. 
  9 
  for 
  Ranina) 
  > 
  

   the 
  decrease 
  in 
  length 
  of 
  the 
  carapace 
  and 
  the 
  increase 
  in 
  width 
  of 
  its 
  

   anterior 
  margin. 
  The 
  mouth 
  is 
  also 
  brought 
  to 
  a 
  more 
  anterior 
  and 
  less 
  

   ventral 
  position 
  than 
  in 
  the 
  Macrura. 
  

  

  