﻿A 
  STUDY 
  IN 
  0AKC1NOLOGY. 
  69 
  

  

  efficiency 
  as 
  organs 
  of 
  natation 
  of 
  those 
  broad 
  blades 
  borne 
  at 
  the 
  end 
  of 
  

   slender 
  and 
  very 
  mobile 
  limbs 
  is 
  apparent. 
  Evidently 
  Raninoides 
  is 
  a 
  

   strong 
  swimmer 
  and 
  a 
  poor 
  digger, 
  and 
  as 
  such 
  stands 
  at 
  the 
  end 
  of 
  

   the 
  series 
  opposite 
  from 
  Ranina. 
  As 
  the 
  posterior 
  branchial 
  orifices 
  arc 
  

   absent, 
  one 
  might 
  expect 
  to 
  find 
  special 
  arrangements 
  for 
  an 
  incurrent 
  

   respiratory 
  stream 
  in 
  the 
  frontal 
  region, 
  but, 
  though 
  I 
  have 
  looked 
  carefully 
  

   for 
  such, 
  I 
  have 
  been 
  quite 
  unsuccessful. 
  However, 
  I 
  will 
  deal 
  with 
  

   this 
  question 
  further 
  on. 
  

  

  Lyreidus 
  (fig. 
  4) 
  exhibits 
  so 
  many 
  resemblances 
  to 
  Raninoides 
  that 
  one 
  

   can 
  hardly 
  doubt 
  that 
  the 
  two 
  are 
  closely 
  related, 
  yet, 
  as 
  I 
  shall 
  show, 
  they 
  

   are 
  clearly 
  adapted 
  to 
  different 
  conditions 
  of 
  life. 
  In 
  Lyreidus 
  the 
  abdomen 
  

   is 
  narrow; 
  its 
  first 
  three 
  segments 
  lie 
  nearly 
  in 
  a 
  straight 
  line 
  with 
  the 
  

   carapace, 
  but 
  the 
  fourth 
  is 
  of 
  peculiar 
  shape, 
  bears 
  a 
  strong 
  median 
  dorsal 
  

   spine, 
  and 
  constitutes 
  as 
  it 
  were 
  the 
  knuckle 
  of 
  a 
  sharp 
  ventral 
  flexure. 
  The 
  

   sixth 
  abdominal 
  segment 
  is 
  long 
  and 
  narrow 
  ; 
  its 
  posterior 
  angles 
  are 
  pro- 
  

   duced 
  into 
  small 
  aliform 
  processes 
  which 
  at 
  first 
  sight 
  might 
  be 
  mistaken 
  

   for 
  vestigial 
  uropods, 
  but 
  they 
  are 
  only 
  processes 
  of 
  the 
  tergum 
  having 
  on 
  

   their 
  ventral 
  surfaces 
  small 
  concavities 
  which, 
  when 
  the 
  abdomen 
  is 
  flexed 
  to 
  

   its 
  fullest 
  extent, 
  engage 
  with 
  small 
  knobs 
  on 
  the 
  two 
  pterygoid 
  processes 
  

   extending 
  backwards 
  from 
  the 
  twelfth 
  sternum. 
  No 
  such 
  apparatus 
  for 
  

   locking 
  the 
  flexed 
  abdomen 
  to 
  the 
  sternum 
  is 
  seen 
  in 
  any 
  other 
  Kaninid. 
  

   The 
  last 
  pair 
  of 
  pereiopods 
  are 
  so 
  slender 
  that, 
  like 
  those 
  of 
  Raninoides, 
  they 
  

   may 
  be 
  described 
  as 
  filiform 
  : 
  they 
  terminate 
  in 
  small 
  flattened 
  elliptical 
  

   dactyli. 
  Correlated 
  with 
  the 
  reduction 
  of 
  the 
  abdomen 
  and 
  of 
  the 
  last 
  pair 
  

   of 
  pereiopods 
  is 
  the 
  absence 
  of 
  posterior 
  branchial 
  orifices. 
  I 
  have 
  studied 
  

   this 
  point 
  carefully 
  and 
  am 
  certain 
  that 
  these 
  orifices 
  are 
  non-existent 
  in 
  

   L. 
  tridentatus, 
  nor 
  could 
  I 
  find 
  any 
  trace 
  of 
  them 
  in 
  the 
  large 
  specimen 
  of 
  

   L. 
  channeri 
  in 
  the 
  Natural 
  History 
  Museum. 
  As 
  there 
  are 
  no 
  posterior 
  

   respiratory 
  orifices 
  the 
  water 
  conduits 
  of 
  the 
  flanks 
  are, 
  as 
  might 
  be 
  expected, 
  

   absent. 
  The 
  epimera 
  of 
  the 
  posterior 
  thoracic 
  somites 
  are 
  nearly 
  flat 
  and 
  

   the 
  edge 
  of 
  the 
  branchiostegite 
  is 
  but 
  slightly 
  prominent 
  and 
  bare 
  of 
  setae. 
  

   In 
  the 
  frontal 
  and 
  oral 
  regions 
  there 
  are 
  also 
  many 
  points 
  of 
  resemblance. 
  

   Though 
  the 
  "front" 
  is 
  truncated 
  and 
  scarcely 
  narrower 
  than 
  the 
  broadest 
  

   part 
  of 
  the 
  carapace 
  in 
  Raninoides, 
  the 
  distance 
  between 
  the 
  extra-orbital 
  

   spines 
  and 
  the 
  lateral 
  spines 
  shows 
  that 
  this 
  region 
  has 
  undergone 
  elongation. 
  

   Further 
  examination 
  shows 
  that 
  it 
  is 
  the 
  antennary 
  somite 
  that 
  has 
  been 
  

   lengthened, 
  for 
  the 
  antennary 
  sternum, 
  which 
  has 
  more 
  or 
  less 
  the 
  shape 
  of 
  an 
  

   equilateral 
  triangle 
  in 
  Notopus 
  and 
  Noto 
  sceles, 
  i* 
  an 
  isosceles 
  triangle 
  iu 
  

   Raninoides. 
  In 
  Lyreidus 
  the 
  elongation 
  of 
  the 
  antennary 
  somite 
  is 
  carried 
  

   to 
  an 
  extreme, 
  and 
  the 
  front 
  being 
  narrowed, 
  the 
  fore 
  part 
  of 
  the 
  carapace 
  is 
  

   produced 
  into 
  the 
  snout 
  characteristic 
  of 
  the 
  genus, 
  the 
  lateral 
  spines 
  being- 
  

   situated 
  far 
  behind 
  the 
  orbits. 
  As 
  I 
  shall 
  -how 
  further 
  on, 
  the 
  details 
  of 
  the 
  

  

  