﻿IKEDALE 
  : 
  THE 
  GASTROPOD 
  APEX 
  IN" 
  CLASSIFICATION. 
  321 
  

  

  (Jrabau 
  (Amer. 
  Nat., 
  toI. 
  xxxvi, 
  p. 
  922, 
  1902), 
  who 
  concluded 
  from 
  

   the 
  study 
  of 
  dissimilar 
  molluscs, 
  "I 
  believe 
  it 
  is 
  not 
  too 
  much 
  to 
  say 
  

   that 
  the 
  protoconchs 
  of 
  all 
  the 
  species 
  within 
  a 
  given 
  genus 
  should 
  

   agree 
  as 
  to 
  their 
  essential 
  characteristics, 
  and 
  that 
  no 
  species 
  can 
  be 
  

   considered 
  congeneric 
  in 
  which 
  the 
  protoconch 
  shows 
  a 
  radical 
  

   difference," 
  and 
  added, 
  "The 
  smaller 
  the 
  taxonomic 
  group 
  the 
  more 
  

   nearly 
  identical 
  must 
  be 
  the 
  earlier 
  stages, 
  since 
  in 
  a 
  small 
  group 
  the 
  

   members 
  have 
  not 
  become 
  widely 
  separated 
  from 
  their 
  common 
  

   ancestor." 
  

  

  ;My 
  researches 
  lead 
  me 
  to 
  endorse 
  these 
  statements, 
  though 
  a 
  recent 
  

   writer 
  (Burnett 
  Smith, 
  Proc. 
  Acad. 
  J^at. 
  Sci. 
  Philad., 
  vol. 
  lix, 
  p. 
  242, 
  

   1 
  907) 
  has 
  arrived 
  at 
  an 
  opposite 
  view 
  from 
  a 
  study 
  of 
  molluscs 
  

   similar 
  to 
  Grabau's 
  specimens. 
  

  

  A 
  splendid 
  instance 
  that 
  would 
  confirm 
  Grrabau's 
  second 
  item 
  is 
  

   that 
  of 
  the 
  Thais 
  succincta, 
  Lamk., 
  group. 
  Thus, 
  the 
  allotment 
  of 
  

   juveniles 
  of 
  a 
  mixed 
  lot 
  of 
  T. 
  succincta, 
  Lamk., 
  T. 
  striata, 
  Mart., 
  

   T. 
  tritoniformis, 
  Blainv., 
  and 
  T. 
  Smithi, 
  Brazier, 
  to 
  each 
  species 
  is 
  almost 
  

   an 
  impossibility, 
  though 
  the 
  adults 
  are 
  easily 
  separable. 
  I 
  only 
  cite 
  

   this 
  case, 
  but 
  various 
  others 
  could 
  be 
  quickly 
  jjroduced, 
  e.g. 
  the 
  

   Trophon 
  Geversianus 
  group, 
  the 
  New 
  Zealand 
  ' 
  Siphonalia 
  ', 
  or 
  the 
  

   New 
  Zealand 
  ' 
  Etdhria 
  '. 
  

  

  It 
  should 
  be 
  carefully 
  noted 
  that 
  the 
  material 
  point 
  in 
  Grabau's 
  

   dictum 
  is 
  the 
  presence 
  in 
  the 
  protoconchs 
  of 
  "a 
  radical 
  difference". 
  

   This 
  is 
  most 
  important, 
  as 
  size 
  generally 
  does 
  not 
  constitute 
  a 
  "radical" 
  

   difference. 
  Moreover, 
  it 
  is 
  rarely 
  possible 
  to 
  absolutely 
  indicate 
  the 
  

   nature 
  of 
  an 
  apex 
  from 
  a 
  casual 
  examination 
  of 
  an 
  adult 
  shell, 
  and 
  

   very 
  often 
  also 
  from 
  quite 
  juvenile 
  shells. 
  

  

  That 
  many 
  species 
  owe 
  their 
  distribution 
  to 
  their 
  free-swimming 
  

   Sinusigera 
  larvae 
  appears 
  to 
  be 
  completely 
  proved 
  from 
  the 
  examination 
  

   of 
  almost 
  any 
  Pacific 
  island 
  marine 
  molluscan 
  fauna, 
  when 
  the 
  

   majority 
  of 
  the 
  species 
  are 
  at 
  once 
  recognizable 
  as 
  possessing 
  free- 
  

   swimming 
  larvae 
  of 
  varied 
  kinds, 
  as 
  Tonna, 
  Septa, 
  Cassidea 
  ; 
  others, 
  as 
  

   Thais, 
  Alectrion, 
  Columhella, 
  Cerithioids, 
  Triphora, 
  Mitra, 
  have 
  just 
  

   been 
  shown 
  to 
  be 
  Sinusigera 
  as 
  juveniles; 
  next 
  would 
  come 
  Strombus, 
  

   Cyprcea, 
  Conus, 
  of 
  which 
  we 
  do 
  not 
  know 
  yet 
  the 
  absolute 
  character 
  

   of 
  the 
  larva; 
  then 
  we 
  have 
  the 
  Solarioids, 
  which 
  we 
  know 
  to 
  travel 
  

   by 
  means 
  of 
  free-swimming 
  larvae 
  of 
  a 
  kind 
  totall)' 
  at 
  variance 
  with 
  

   the 
  preceding, 
  namely, 
  that 
  peculiar 
  pseudo-sinistral 
  mollusc 
  described 
  

   as 
  ' 
  Agadina' 
  ; 
  lastly, 
  shells 
  which 
  though 
  having 
  a 
  wide 
  range 
  

   apparently 
  give 
  no 
  clue 
  to 
  their 
  means 
  of 
  distribution. 
  

  

  The 
  'Agadina' 
  stands 
  clearlj' 
  revealed 
  when 
  such 
  a 
  widely 
  urabili- 
  

   cated 
  shell 
  as 
  Heliacus 
  crenellus, 
  L., 
  is 
  observed 
  : 
  it 
  is 
  not 
  so 
  easily 
  seen 
  

   in 
  a 
  species 
  like 
  Architectonica 
  cingulum, 
  Kiener. 
  It 
  occurred 
  to 
  me 
  

   that 
  it 
  would 
  scarcely 
  be 
  noticeable 
  in 
  an 
  imperforate 
  shell, 
  the 
  sinistral 
  

   apex 
  being 
  then 
  the 
  only 
  clue. 
  Whilst 
  considering 
  that 
  this 
  might 
  

   be 
  the 
  solution 
  of 
  some 
  of 
  my 
  puzzles 
  I 
  stumbled 
  upon 
  an 
  '■ 
  Agadina 
  ' 
  

   commencing 
  a 
  post-embryonic 
  whorl. 
  It 
  was 
  obvious 
  the 
  shell 
  was 
  

   not 
  a 
  Solarioid, 
  and, 
  moreover, 
  that 
  it 
  was 
  going 
  to 
  be 
  imperforate 
  of 
  

   a 
  peculiar 
  kind. 
  I 
  could 
  not 
  place 
  it 
  anywhere 
  near 
  the 
  Solarioids, 
  

   the 
  only 
  group 
  to 
  which 
  '■Agadina'' 
  had 
  hitherto 
  been 
  traced. 
  The 
  

  

  