﻿98 
  PROCBEDINGS 
  OF 
  TlIK 
  MALACOLOGICAL 
  SOCIETY. 
  

  

  This 
  type 
  of 
  radula 
  occurs 
  in 
  the 
  following 
  species 
  only: 
  — 
  

  

  haustrum, 
  Mart. 
  : 
  New 
  Zealand. 
  

  

  laamosa, 
  Brug. 
  (= 
  striata, 
  Mart.): 
  New 
  Zealand. 
  

  

  scohina, 
  Q,uoy 
  (+ 
  vars. 
  albomarffinata, 
  Desli., 
  iristis, 
  Dunk.): 
  New 
  

   Zealand. 
  

  

  adelaidensis, 
  Crosse 
  (" 
  Ricinula^'): 
  Southern 
  Australia. 
  

  

  It 
  marks 
  them 
  off 
  in 
  a 
  decisive 
  manner 
  both 
  from 
  the 
  27iais 
  proper 
  

   and 
  from 
  the 
  Nucella 
  group, 
  Avhicli, 
  with 
  ^heir 
  tricuspid 
  rhachidian 
  

   and 
  numerous 
  minor 
  denticles, 
  stand 
  in 
  much 
  closer 
  relationsliip 
  to 
  

   one 
  another 
  than 
  either 
  of 
  them 
  do 
  to 
  the 
  haustrum 
  group. 
  At 
  the 
  

   same 
  time 
  one 
  is 
  inclined 
  to 
  think 
  that 
  there 
  is 
  a 
  distinct 
  analogy 
  

   between 
  the 
  four 
  side 
  cusps 
  in 
  this 
  group 
  and 
  the 
  two 
  side 
  cusps, 
  

   each 
  with 
  its 
  strong 
  interior 
  denticle, 
  in 
  Thais 
  proper. 
  It 
  seems 
  

   reasonable 
  to 
  suppose 
  that 
  the 
  two 
  exterior 
  cusps 
  (let 
  us 
  call 
  them 
  

   cusps 
  1 
  and 
  5, 
  reading 
  from 
  the 
  left 
  when 
  the 
  cusps 
  point 
  away 
  from 
  

   you) 
  in 
  the 
  haustrum 
  group 
  are 
  analogous 
  to 
  the 
  two 
  exterior 
  cusps 
  

   in 
  the 
  Thais 
  group, 
  while 
  the 
  interior 
  denticle 
  which 
  is 
  attached 
  to 
  

   those 
  cusps 
  in 
  the 
  latter 
  group 
  represents 
  cusps 
  Nos. 
  2 
  and 
  4 
  in 
  

   the 
  haustrum 
  group, 
  the 
  proximity 
  of 
  tlie 
  cusps 
  1 
  and 
  2 
  and 
  of 
  

   cusps 
  4 
  and 
  5 
  being 
  significant 
  when 
  regarded 
  in 
  this 
  light. 
  It 
  is 
  

   conceivable 
  that, 
  in 
  earlier 
  representatives 
  of 
  the 
  group, 
  now 
  extinct, 
  

   in 
  proportion 
  as 
  the 
  attached 
  denticle 
  became 
  larger, 
  it 
  pushed 
  the 
  

   cusp 
  to 
  which 
  it 
  adhered 
  away 
  from 
  the 
  central 
  cusp, 
  until 
  it 
  

   established 
  itself 
  in 
  the 
  place, 
  and 
  with 
  the 
  dimensions 
  it 
  now 
  holds 
  

   in 
  the 
  haustrum 
  group. 
  Or 
  the 
  reverse 
  process 
  may 
  have 
  occurred, 
  

   and 
  the 
  New 
  Zealand 
  group 
  may 
  be 
  the 
  progenitors 
  of 
  the 
  27tais 
  group, 
  

   cusps 
  2 
  and 
  4 
  becoming 
  smaller, 
  drawing 
  nearer 
  to 
  cusps 
  1 
  and 
  6, 
  

   and 
  finally 
  coalescing 
  with 
  them 
  in 
  the 
  form 
  of 
  the 
  denticle 
  now 
  

   seen 
  on 
  their 
  inner 
  side. 
  

  

  haustrum, 
  Mart. 
  Central 
  cusps 
  decidedly 
  longer 
  than 
  the 
  four 
  side 
  

   cusps, 
  all 
  the 
  cusps 
  sharply 
  pointed, 
  and 
  equidistant 
  from 
  one 
  

   another, 
  cusps 
  1 
  and 
  5 
  broader 
  than 
  2 
  and 
  4 
  ; 
  knob 
  fairly 
  prominent; 
  

   base-line 
  hardly 
  curved, 
  angles 
  of 
  the 
  base 
  decidedly 
  produced. 
  

   Troschel's 
  figure 
  is 
  accurate, 
  except 
  that 
  the 
  central 
  cusp 
  is 
  not 
  long 
  

   enough. 
  (Fig. 
  23.) 
  

  

  lacunosa, 
  Brug. 
  Central 
  cusp 
  decidedly 
  the 
  longest, 
  all 
  the 
  

   cusps 
  rounded 
  rather 
  than 
  sharp 
  at 
  the 
  apex, 
  knob 
  scarcely 
  showing, 
  

   base-line 
  nearly 
  straight, 
  not 
  indented, 
  angles 
  of 
  base 
  slightly- 
  

   produced, 
  

  

  scohina, 
  Q,uoy. 
  Central 
  cusp 
  somewhat 
  sharp, 
  side 
  cusps 
  shorter 
  

   and 
  blunt; 
  cusps 
  1 
  and 
  2, 
  and 
  4 
  and 
  5, 
  are 
  close 
  together 
  and 
  well 
  

   away 
  from 
  the 
  central; 
  knob 
  prominent; 
  base-line 
  waved 
  in 
  the 
  

   centre; 
  basal 
  angles 
  scarcely 
  produced. 
  The 
  same 
  points 
  recur 
  in 
  

   the 
  varieties 
  alhomarginata, 
  Desh., 
  and 
  tristis, 
  Dunk. 
  (Fig. 
  24.) 
  

  

  adelaidensis, 
  Crosse. 
  The 
  cusps 
  are 
  somewhat 
  sharper 
  than 
  in 
  

   scohina, 
  and 
  the 
  knob 
  is 
  decidedly 
  more 
  prominent; 
  the 
  base 
  is 
  

   slio-htlv 
  waved 
  in 
  the 
  middle 
  and 
  the 
  base 
  angles 
  are 
  scarcely- 
  

   produced. 
  Cusps 
  1, 
  2, 
  and 
  4, 
  5 
  are 
  close 
  together. 
  In 
  other 
  

   respects 
  tlie 
  form 
  of 
  the 
  radula 
  corresponds 
  closely 
  to 
  that 
  of 
  

   scohina. 
  (Fig. 
  25.) 
  

  

  